Differences in the way they play
Dark Heresy is probably the most similar to a traditional game in this regard. Because there is no default status quo, you can pretty much go in any direction imaginable without significant hand waving.
Rogue Trader does tend to lend itself to games that have some space combat/travel/exploration component in order to allow all players to contribute (not quite as bad as deckers in Shadowrun, but a similar problem).
Deathwatch really and truly seems designed for episodic play (though you could easily work in overarching plots). A lot of the rules necessary for longer play (insanity and corruption in particular) are very weak and not well developed and the mission-based structure essentially gives you the format for each episode.
Any reason to recommend one over another
I think each game really does bring something unique to the role playing experience. To some degree, these answers are quite subjective and could be modeled in other games, but the rich backdrop of the 40K universe really helps drive these elements home.
- Dark Heresy - Really encourages the exploration of the mythos and gives a GM a lot of opportunity to develop deep, intricate (perhaps even obtuse) plots with encounters that seem truly lethal and require cunning and skill to get through.
[rich skills, winding plots, lower power level increases lethality]
- Rogue Trader - Out of the systems, this is the most interesting in a lot of ways in that it delivers on the promise of roleplaying in a Star Trek style without that silly Prime Directive. Players are fairly powerful and masters of their own fate, each highly specialized with something to do. Combined with the geeky attraction of designing your ship, I think this game has quite a bit to offer.
[Star Trek-style exploration, deep character specialization, ship design!]
- Deathwatch - For many, this game will be the winner simply because it's Space Marines. While it does lend itself to combat-intensive sessions, FFG has done a good job of trying to give the marines some actual personality (although this personality is often hidden and only revealed in the solace of their own rooms). However, the mission structure has a similar appeal to the job layout of Shadowrun games and can really help get players moving. And of course I would be remiss if I didn't mention the very detailed combat mechanics and options which do allow for some fairly cinematic gameplay.
[roleplaying hints for players, clear session objectives, interesting combat]
Any with a more enthusiastic following than another
I think it's safe to say that Dark Heresy has a larger following, but this is a factor of age. Otherwise, I think the vibrancy of all three games is pretty light outside of Fantasy Flight Games' own forums (though I did notice that the Deathwatch forum nearly has as many posts as the Rogue Trader forum at this point even with the year head start the game had, so it may be that Deathwatch will end up being the more popular game).
You've made two key changes:
- Allowing multiple combat half-actions per round
- allowing movement to be replaced by a non-movement half-action
The primary factors I see are:
- increase in Psyker offensive capability (Most of their offense is more powerful than their allowed weapons; it gets worse under RT or DW, due to a different psychic system)
- decreased access to defenses¹
- decreased emphasis on movement²
- increased emphasis on non-action defense (IE, cover)³
- breaks the equipment bonuses
- decreases incentives to use burst and full-auto autofire modes
- cybernetics and talents with extra half-actions become devalued⁴
- initiative becomes more important⁵
- Ganging up becomes less essential⁶
¹: Since one gets very few defenses per turn, this change of yours makes them less valuable, by allowing them to be overwhelmed faster.
²: The current system makes movement a valuable and essentially irreplaceable part of the round; take it or not, it's lost if unused or unusable. It's not terribly realistic, but is very cinematic. And DH, RT, and DW are all intended to be very cinematic in tone.
³: At present, especially for melee, active defenses (dodge and parry) are quite valuable; cover is less so, but not enough to render it useless. When one allows replaceing movement with a second attack, cover suddenly becomes MUCH more attractive, as it's an assured penalty to be hit, instead of a roll by the defender... but it also reduces drama.
⁴: At present, the only way to get a second attack is expensive talents and/or cyberware with inexpensive talents. If a character can just hole up in cover and double shoot, these talents are far less valuable, making the Assassin and Guardsman's additional combat actions less useful.
⁵: Further, by allowing the etra attacks, you can run a target out of actions sooner, as well, so initiative becomes more important to prevent having actions drained in defenses.
⁶: Ganging up is the great equalizer in RT and DH combats; it's how one overwhelms defenses. Given that every character can defend once for free, and once by aborting, if you can attack twice, you've just eaten both defenses this round, meaning you only need one attack from a buddy to get through. The norm is that you need two buddies to negate the defenses, or one buddy with a multiple attack talent.
It sounds like you made these changes with rank 1-2 characters... it's going to be more profoundly off-norm or higher rank characters, as normal slow growth of actions is made to feel even slower by providing far less of a bonus over not having them.
Best Answer
In Deathwatch rpg you measure how well you do things against others by comparing degrees of success in what the rules calls an opposed test. For every 10% you roll below or above your percentage you gain one degree of success or failure. So if your character with WS 70 roll 47, he has 2 degrees of success (70-47=23, use only the tens). If your opponent dodges and rolls 51, he has one degree of success (70-51=19). His degree counters one of your two successes, but you still have one left so you hit as his dodge wasn't good enough.
Unfortunately the Deathwatch rule book does not state that dodge is an opposed test, but in other 40k rpgs this is changed. So I suggest you do the same.
All of the 40k rpgs use degrees of success in their rules, but with small variations when it comes to how you calculate them. In the newer ones (Black Crusade and Only War) you gain one degree by succeeding with your roll and add 1 degree for each 10% you roll under. So in the example above your character would have 3 degrees of success and your opponent 2.
Hope this helps.:)