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