I don't think there is a right answer to this. Play on a server close to your location and have at it. Someone released some statistics that showed all factions are very balanced. Play around with everything and see what you like.
Personally, I play a lot of heavy assault with some medic so I've started to cert my heavy assault more. I'm awful at flying and only use tanks for a change of pace.
I really enjoy the game though, so easy to play in small chunks but can keep me entertained for hours if there is a good battle going.
Thanks for the info, I downloaded the game last night and watched some of the tutorial videos, then played till about 3 in the morning
I joined the west coast server and currently playing as NC medic. I had a hell of a time with the game crashing to desktop, but verifying game files via Steam seemed to help. Guess I should look around for more tweaks/stability improvements