I too am facing a similar issue. We have about 5 core guys though, with some others coming in and out as they make sessions, and I think our first 4 guys went Operator, Ratling, Medic, Stormtrooper. This left me waffling between a Pysker, a command character like a SGT or my two preferred RP choices, the Ogryn or the Priest. I've always loved Klovis the Redeemer, so I decided to make a crazy redeemer character even though the Regiment the guys created is 100% useless to him for talents and skills.
However, we don't start the main campaign for like another week, and I just can't shake the feeling everyone else is going to be kicking ass and taking names and I am going to die horribly the first encounter.
I looked long and hard at the SGT and the Commissar (Because no one had stepped up to command the squad yet), but just couldn't get a feel for how you build them. However, looking at every class, I feel like the Heavy Gunner, Weapon Specialist, Ratling and Stormtrooper synergize out well, The Ogryn to a lesser extent though limited by 4 weapons choices and talking like a brick. Operator and Medic seem like decent combat guys with useful side jobs, the TechPriest seems like a walking death machine if your GM gives you access to all the cybernetics and stuffs. Pyskers, I am not sure how they don't kill themselves the 4th time they cast an unfettered power. Priest I haven't figured out at all, no real synergy in his aptitudes and Melee seems like a bad place to be about 85% of the time. Just looking at an Ork, an ork hits you twice, you are rolling a new character, and the XP you need to get "decent" seems a bit high. Flame weapons can crutch him a bit, but he just seems odd to me, like they had a great idea, then made a melee character with access to two aptitudes for social skills.
Don't know where I was going, started rambling....anyway, I feel you, game system seems built well enough that most classes are a good option. I guess I haven't played enough to fully understand the power of the "Leader" type classes or the Ministorum Priest. My 2 cents, if you want to kick ass go Heavy gunner? As a straight butt kicker it seems like a great choice.