Warmachine/Tactics/Differences between Warmachine and 40k
Overarching Differences
In terms of metagame differences, Warmahordes games tend to be much shorter and less time-consuming than Warhammer. It's also much cheaper to build an army for, and therefore makes you less of a tool for trying to get your friends into it.
As for the game itself, Warmahordes is generally more comparable to Warhammer Fantasy than 40k. The biggest comparison that could be drawn between them is that Warmahordes loves melee. Gunlines exist (see Cygnar), but shooting is generally a supplemental option rather than the bulk of your army. Ranged units exist to soften targets up for the melee heavy hitters, rather than to wipe entire squads from the board. Warmahordes armies also love to charge the enemy, and the game will often swing on one side or the other finding a better time to run screaming into the enemy's face. Aggression is key, to the point that the game's official tagline is PLAY LIKE YOU GOT A PAIR!
Warmahordes also places a greater emphasis on your command units than Warhammer. In most scenarios, losing your warcaster or warlock means instant defeat, as if your opponent had taken your king in a game of chess. Even in games where it doesn't mean instant loss, it usually means definite loss, because your warjacks will shut down and your warbeasts go crazy if the warnoun controlling them suddenly evaporates. Warmahordes armies have to strike a balance of aggression and defense in order to inflict crippling damage while preventing an embarrassing defeat from an unexpected bombing run on that squishy wizard in the back.
Warcasters and warlocks influence the game even more hugely than that, though. A given army's playstyle is in large part defined by which warnoun is commanding it, since the spell list in play is so vital to which strategies you can expect to execute well. Each warnoun also comes with a "feat" ability, which is a once-per-game super-awesome magical nuke effect unique to them. These range from freezing enemy models in place to setting everything on fire to outright denying your opponent their allotment of focus next turn, and are some of the most devastatingly powerful abilities in play. Warmahordes players must know their warnoun inside and out to make proper use of them, and they'd damn well better know the opponent's, too.
Warmahordes battles also tend to be smaller in scale and more tactically focused than Warhammer ones. There are fewer models in play, and they all have greater freedom of movement than their Warhammer counterparts. Carefully positioning individual members of a unit to block vital charge lanes is a skill that all Warmahordes players must learn pretty damn quick, and it isn't uncommon for different members of a single unit to be attacking multiple different targets.
Cost and Models
A major difference is the model count and real money cost of the games. Whilst a proactive buyer may be able to get a competitive Warhammer army from ebay for fairly cheap (talking maybe $300), an army that is ready to play can cost as little as $60 for Warmahordes, although a proper 35 point game will more likely set you back around $150-250. In Warmahordes the typical model count can vary between 4-50 whereas in Warhammer that can be in a single unit. Warmahordes is much more favourable to variety than Warhammer such that almost all units are viable one way or another and in the case of most armies if two different people bring the same faction their lists will likely not even share a common unit. In Warhammer however many races have must take units and sometimes even have a specific list to take if the player wants to have a chance in a tournament.
Also in Warmahordes by and large there is NO CUSTOMIZATION. With many light to medium units you can choose whether you want a smaller or a larger unit e.g. 4 OR 6 and with many units you can choose to take a Unit Attachment which offers a variety of bonuses. Aside from this there is NO CUSTOMIZATION. You cannot swap out a model's weapon, you cannot elect to drop a special rule for a point consolidation and you cannot deck your HQ out with sick ass weapons, in many ways this is by design to aid with balancing.
Rolling
Finally a major difference is the way the dice are rolled in Warmahordes. In Warhammer almost all rolls go off a 1d6 system which is an even distribution on each individual roll which can cause crippling clusters of random number generation when very few dice are being rolled. The system works well with units that have large numbers of dice rolls such as Ork or Tau rank and file but becomes risky at best when rolling important rolls for expensive models such as losing the blast from a Riptide on the gets hot roll. Warmahordes has somewhat mitigated this risk (thus enabling less infuriating gameplay with a low model count) by utilizing a 2d6 system on almost all dice rolls and often having the option to "boost" rolls by adding an extra d6. Using the cumulative total from multiple dice on each roll gives a rough approximation of a normal distribution making tactics in general more reliable.
Additionally in Warmahordes all rolls (ranged and melee) to hit are made against the target's defence. This is contrast to Warhammer where the to-hit roll in shooting was made despite the target's stats, taking only the shooter's BS and terrain effects like cover into account. The end result is that in Warmahordes a model's survivability is not purely dependent on its heath boxes or armour but also on its defence (difficulty to be hit).
Wounds/Boxes and Armour
In Warhammer a typical model has 1 wound with the toughest of models having around 5-6, generally speaking (in the absence of special rules) only 1 wound may be taken of by each attack or shot. In Warmahordes most light and medium infantry have only a single wound (wounds are referred to as boxes in Warmahordes), heavy infantry and special characters have either 5 or 8, warcasters/warlocks have between 14 and twenty (with Karchev the Terrible being in a class of his own), and most warjacks or warbeasts have in the range of 25-36, working more like video-game health than wounds in Warhammer. Warbeasts and warjacks can also be damaged or wounded if enough of their "boxes" are used up, reducing their combat or utility abilities in future turns unless repaired or healed but allowing them to remain on the field.
In Warhammer 40k, armour (and various derivatives) works as a set armour "save" roll that the model either gets or does not. A success means the model lives with no penalty and a failure means the model either dies or loses a wound. In Warmahordes armour is not rolled as a "save," instead functioning more like a Warhammer "Toughness" stat, though without requiring the use of a chart. To deal damage, the attacking model rolls an additional 2d6 after determining whether the defending model was hit, adding the result to the "power" of the attack. The defending model will then take the difference in damage. A special rule, "Tough" works similar to a Warhammer armor "save," but it only works on a 5+ and only if the model is about to die, making it much less reliable overall.
Activation
In Warhammer all units activate in phases: everyone in the army moves, then all units shoot, etc. In Warmahordes each unit (or individual model for those outside of units) takes its full activation individually. For instance, if the player chooses to first activate a warjack, that warjack will take its movement and then its action, fulfilling all of its special rules before completing its turn. If a unit of 10 models is activated all 10 models will move together and then all 10 models will take their actions together completing their turns. This means that the order in which models are activated generally makes a much larger difference than it does in 40k.
Finally, in the absence of special rules, models may only make attacks in their own turn in Warmahordes as opposed to 40k where all engaged models make attacks in melee combat. Think Dungeons & Dragons rules and you won't be far off.
Typical Stats and Gameplay
Please note everything in this section will be horribly oversimplified for experienced players and is designed to merely give newbies an idea of the gameplay differences. Obviously all of these stats will vary depending on what sort of niche the unit fills. Generally you can assume values higher than those shown are above average and lower are below average.
Warhammer 40k
The typical troop in 40k will have a statline something like this: Weapon Skill 3-4, Ballistic Skill 3-4, Strength 3-4, Toughness 3-4, Wounds 1, Initiative 3-4, Attacks 1, Leadership 7-9, Armour Save 4-5
Whereas a big beasty might have a statline like this: Weapon Skill 3-4, Ballistic Skill 3-4, Strength 6, Toughness 6, Wounds 3-4, Initiative 3-4, Attacks 4, Leadership 7-9, Armour Save 4-5
Vehicles: A vehicles statline works the same except that is has a armour value instead of toughness or an armour save, often has no attack value or initiative and generally moves faster.
A typical range of shooting weapons in 40k is 24".
In the absence of special rules all models move 6 inches.
40k Gameplay
In 40k a model will move in moving phase (generally 6") and then shoot in the shooting phase. The shot will go something like this:
1. Pick a target in range.
2. Roll to hit with a higher chance if the model has a high BS. If the attack misses nothing further.
3. Roll to wound with a higher chance if the strength of the attack was higher than the toughness of the target. If the attack doesn't wound nothing further.
4. The weapon used will either pierce right through the targets armour in which case the target takes a wound and nothing further.
5. Else the weapon does not pierce the targets armour in which case the target rolls an armour save with a higher chance of living if it has a LOWER armour save value. If the the target makes the save nothing further.
6. Else the target takes a wound and nothing further.
7. Depending on how much damage the target unit took it may need to pass a leadership test by rolling lower that its Ld on 2d6 or run away.
Whereas a melee round will go something like this:
1. Two units are in melee.
2. The one with the higher Initiative goes first.
3. Make as many attacks as the models Attack stat.
4. Roll to hit with a higher chance to hit if the models WS is higher than the target. If the attack misses nothing further.
5. Roll to wound with a higher chance if the strength of the attack was higher than the toughness of the target. If the attack doesn't wound nothing further.
6. The attack will either pierce right through the targets armour in which case the target takes a wound and nothing further.
7. Else the weapon does not pierce the targets armour in which case the target rolls an armour save with a higher chance of living if it has a LOWER armour save value. If the the target makes the save nothing further.
8. Else the target takes a wound and nothing further.
9. The unit with the lower initiative makes its attack as described above.
10. The unit that took the most wounds needs to pass a leadership test by rolling lower that its (Leadership - (wounds taken - wounds given)) on 2d6 or run away. Example unit 1 has Ld 8, dealt 7 wounds and took 5. It must roll lower than 6 on 2d6. 6 because its Ld of (8-(7-5))=6.
Against vehicles instead of rolling against toughness d6 is added to the strength of the weapon, if the total is higher than the vehicle's armour the vehicle is damaged.
Warmahordes
The typical troop in Warmahordes will have a statline something like this: Speed 5, Strength + Power 11, Melee Attack 5, Ranged Attack 5, Defense 12, Armour 14, Command 8
Whereas a big beasty might have a statline like this: Speed 5, Strength + Power 14, Melee Attack 5, Ranged Attack 5, Defense 11, Armour 18, Command 8
A typical ranged weapon in Warmahordes can shoot 10".
It's worth noting that because Warmahordes uses 2d6 1 point difference in stats is a much less dramatic difference than in 40k.
Warmahordes Gameplay
In Warmahordes a unit will be activated and then takes its full move (generally its speed) and then make its action. A shooting action will look something like this, note shooting attacks just use power, melee uses strength+power:
1. Pick a target.
2. Measure range, if out of range nothing further.
3. Roll to hit 2d6+RAT. If equal to or higher than the target's defence; hit. Else nothing further.
4. Roll to damage 2d6+Power if the damage is higher than the target's armour the target takes the surplus in damage, else nothing further.
Whereas a melee round will go something like this:
1. Two units are in melee.
2. The one who's turn it is may attack, units may not attack outside of their turn.
3. Make an attack with each of the models weapons, usually 1 with smaller models, usually more with larger models.
4. Roll to hit 2d6+MAT. If equal to or higher than the target's defence; hit. Else nothing further.
5. Roll to damage 2d6+Power+Strength if the damage is higher than the target's armour the target takes the surplus in damage, else nothing further.
Summary
In general Warmahordes is a smaller, (in the technical sense) less complex game that requires much less money to start. However, while Warmahordes may have a simpler basic rule set, it has many more special rules, which more than balances this out in terms of depth. Warmahordes is more melee-focused than 40k and games go for a shorter period of time. Finally the 2d6 system allows for a more predictable outcome of any move, which results in a game that often feels less chaotic and random than many Warhammer fights.
However Warmahordes is much less customizable than 40k, so making an army of your dudes is much harder. You can't make your own commander with a history all of his own unless you want to use one of the warnouns that already exist as an identical base. Also, the lack of options for the troops makes making whatever fluff you have for your army more difficult to crunch. Warmahordes feels more like a historical game than a fantasy one in that you're not "forging a narrative" so much as stepping into someone else's. Further, Privateer Press has pretty tight conversion guidelines for its tournaments, (which, to be fair, makes sense because a lot of things like the warjacks share the same chassis and differ only in the arms so making sure no-one is confused is important), limiting how wildly you can convert your models if you want to be competitive.
There are two ways to view 40k VS Warmahordes. If you want a casual game, Warmahordes is far superior for these reasons, but if you are prepared to dump a fair amount of time and money for a larger scale game with a more battlefield commander feel, you may prefer Warhammer. Additionally, Warmahordes' greater predictability places more importance on tactics and is a bit better balanced, with fewer stand-out horrible choices like Pyrovores, making it better for people who want greater tactics in their game. Furthermore, as has been proven with monstrosities like Screamerstar, the Daemon Summoning Tide, and Fish of Fury, 40k is easily broken when taken to high level tournament play with Von, und precisely von vay! to play each of the top level armies.
On the other hand, the 40k ruleset (especially with new Unbound rules) favors "forging a narrative," with players expected to play the army they want with the backstory they want, adding RPG-character elements to the wargame. This is why 40k's plot just keeps treading water - it sets the stage for your dudes to have epic adventures and stories, while Warmachines' plot advances because players want to see what happens next. Historical games like Flames of War or Bolt Action do the same, but as the events are real it's easier to motivate players to play a force from one point of time. If you want to have a good story, go to Warmahordes, but if you want to make a good story, 40k might prove a superior choice.
It's hard to say which is better, since they're trying to be different things, and the answer is very much up to personal taste. Thankfully, because they're trying to be different things and not really copying each other, there's easily enough room for both of them.