It really depends. I did 2x Commandos. I ran a gunnery spec and a hybrid gunnery/medic spec. The gunnery/medic spec was really good for PvP, but gunnery is a little too one trick ponyish for me. Assault is supposed to be really good as well, I haven't run it myself.

Powertech on the other hand has radically different playstyles depending on how you talent yourself out. You can become melee heavy with charges, or you can become more shooty, or you can also run the Firebug tree which is shared.

I'd personally probably run the Powertechs because I think they would be super diverse, and a lot of fun.