Depends how you prefer to play a lot too. I like the mobility of being infantry, but tanks are usually more fun to play hehe. Also with a tank, I can pick engineer and repair my tank between fights or when I retreat while as infantry you cant repair yourself. As infantry I play mobius/mendoza or ravenshaw/the ion cannon chick a lot if I can afford them. It really depends on what kind of role you wanna play I guess. If you wanna hunt down tanks IMO you're best off as ravenshaw/ion cannon (especially the ion cannon one because NOD tanks cant take too many hits). Or you can go sniper to take out their assault infantry, or you can go engineer to repair your buildings/tanks or go take out their tanks (plant timed C4's on them, however a good player will most likely not let an engineer get close) or their buildings. Usually I go with tanks and try to convince others to rush to take out their AGT/Obilisk, and after that I go in with infantry carrying a beacon while the rest holds tank battles.
There's no set rule on what to play, try out the tanks/classes and learn what is good for what job, some classes are better anti tank, some are better anti infantry (some are good against both). So figure out what your team needs and play that class.
[ March 03, 2002: Message edited by: Diablo198 ]