So I am a fairly inexperienced GM. I have ran a handfull of sessions for DnD 4th ed, Pathfinder, Star Wars Saga Edition, and dabbled in various other systems. All in all, I have much more experience READING about these games than I do actually playing them due to a lack of other players to game with.

At this point I more or less have a group to play with online which has driven me to get back into RPGs and Warhammer 40k just happens to be my favorite fictional universe so I really want to try running a game in this setting. So here is the dilemma, rather than something like DnD or Pathfinder where I can more or less just say go kill goblins and call it a day, 40k is a much more defined universe and I am not very confident in my ability to accurately convey the world and its people to my players whether it be roleplaying an inquisitor, or telling them what a hive city is like.

Another issue, arguably the bigger problem, is I have no idea what system to run (DH, RT, DW, BC, or OW). I think I can safely count out Black Crusade, but all the other systems have things I can see my players enjoying. In my group I have one person who enjoys Roleplay over combat and likes mysteries, another who prefers a balance and is somewhat of a power player (always tries to be as overpowered as possible), and the other 2 I am just unsure what they like, but I have found it is a lot harder to force your group to roleplay and investigate then just have them shoot stuff.

If I had it my way personally, I would probably pick Dark Heresy, as it offers a lot of roleplay potential and freedom to make your character unique. I do know however a lot of my group likes the idea of being in the Imperial Guard so Only War would be good, I just worry that Only War would be combat, combat, and more combat, 9/10 times. Deathwatch is cool soley because its the only system your characters arent liable to die from a stray flashlight shot. and while Rogue Trader sounds cool, I get the feeling a sandbox style game is not a good idea for my group, but maybe I am just overthinking it.

TL;DR - I am a noob at GMing but I really wanna run 40k. I don't know how each system works in gameplay so I am worried I will pick a system not suitable for my group. Specifically, how much oppurtunity to roleplay is there in a game like Deathwatch or Only War. Which systems would you say are easier to run, basically looking for general advice on GMing in general, in the 40k universe, and a run down of what each system offers in terms of gameplay and style. (RP vs Combat)

Sorry for the wall of text haha

So in general the rules of the game have been more refined with each game. One problem is that the setting of the game is pretty hard-baked in to the rules, so running an DH-style game using OW rules as written is going to take some shoehorning. If you're feeling adventurous, you can try out the just-published-today second version of the Dark Heresy 2 beta, which has rules in line with OW with the setting of DH. If not, I would suggest picking up the rulebook for whichever flavor game you think your group would enjoy (make sure to download the errata).

All of the systems have pre-published adventures written for them, one included in each of the core rulebooks. Start there to get a feel for the system. The DH one took my group 4-5 sessions to get through and they took it WAY off the rails. In general they're good for fleshing out a setting and its characters.

DW and OW are games focused on combat and their rules reflect it. You can try to make it work in another setting, but in my experience groups will gravitate more toward the game's stock setting. Don't necessarily count out BC, either. It's not like playing Evil characters in D&D - it (can be) more about fighting for what you believe in even though the Imperium disagrees with you. Freedom fighters or something. It's the system where you start as the underdog and try to come out ahead.

DH was the first system I ever GMed anything for. To make it work you definitely need to think on your feet and be good at improv simply because the players are given the most room to act. DH is the oldest system with the least polished mechanics, but your power-gamer player will love the shenanigans he can get away with in it.

If you're coming from D&D, RT is basically life in 40k plus adventurer logic. You get to command a ship and fly around and cause mischief.

A wall of text right back at ya. My advice to you is to choose the system whose setting you think your group will most enjoy and run that. Run the intro adventure in the rulebook so everyone gets a handle on the system, then do whatever. Play a pre-written one or write your own or just totally wing it.

Thanks a lot for all the great advice, I will have to consult with my group more it seems before I make any decisions. When I say I am inexperienced, its mostly because ive only ran probably 10 sessions worth of games so far, and I have used premade material almost exclusively, however I have come to realise, regardless of the system, good improv is always better than having everything set in stone, which improv is probably my weakest skill. I understand coming up with a skeleton of a story and having my players go at it, but I feel like I wouldn't be able to keep up with them, or even properly lead them to a suitable end point, especially when it comes to a mystery plot, I feel like any clue I would leave them would wind up being way to obvious or something.

Well, you could always get started with something like the adventure already in the Dark Heresy Core Rulebook, or the free Introductory Adventures you can download here and here as a PDF. You can modify them to suit whatever your group comes up with, for example to give it more of an Imperial Guard flair. Or just take some inspiration from them! Eventually you ought to get the hang of it.

Oh, and if you do end up with Dark Heresy, do yourself a favour and get both the Core Rulebook as well as Inquisitor's Handbook. The latter not only offers a lot of new tools for your players, but also some more advice for you as their GM!

Awesome, I'll check all of that out

The premades are great to cut your teeth on a learn the system. They are a little too "on rails" for most people's tastes, but they can be modified to solve that problem. Either way, give one of the above a whirl and then write your own.

Let me start with a mandatory link to my combined house rules http://community.fantasyflightgames.com/index.php?/topic/94048-40k-unified-rpg/#entry915872 because it sounds like it could help with not having to choose a system

Moving on though, can I recommend something a little different? Head off the reservation. Rather than a traditional Dark heresy game, why not start with something like an Underhive game, or an Arbites squad. If your strength as a GM is your love for the universe, share that with the group. If you can share your enthusiasm to the group, they'll all want to explore the universe with you and THAT is where you've got a common link with your players.

I would suggest against Deathwatch/Only War unless you really, really want to go combat-heavy. I like Deathwatch a lot but much prefer the investigative feel of Dark Heresy or the "super-Firefly" of Rogue Trader. And I feel like DH/RT are much more broadly inclusive of vastly different character types/ideas.

I would suggest against Deathwatch/Only War unless you really, really want to go combat-heavy. I like Deathwatch a lot but much prefer the investigative feel of Dark Heresy or the "super-Firefly" of Rogue Trader. And I feel like DH/RT are much more broadly inclusive of vastly different character types/ideas.

Yes, this is an excellent advantage of DH and RT-- they provide framework for more expressiveness of story and characters.

The premade adventure in DH2 Beta is actually quite good as an introduction, since it offers the option of three different organizations being either loyal, criminals, or heretics.

Thus the GM can run the campaign quite a few different ways, with much less 'railroad'.