Pistol Marine for life!
I preferred Psy-Ops, and I recall maxing both Pistols and Energy Weapons.
Those psionic upgrades were lifesavers, but the early game is MUCH more difficult than it is for Marines. Impossible IS damn near impossible.
For those who haven't played it, unlike most current FPS-RPG hybrids, you can't actually use certain weapons or abilities without basic training, which consumes upgrade modules... meaning you have to really think about what upgrades you need (I always went basic weapons -> hacking -> psionics -> advanced weapons). Defense, resource collection, abilities, offensive carnage, in other words.