Sniper, because they are used for precision making shots, 1 shot, 1 or more kills. It's like boxing, you can throw little jabs and hooks to add up damage, or you can get in there and be patient, wait for the moment and throw the perfect punch, kick, knee ect. and the fight is over.
since there is no jets.. then id go assult.. i wanna be able to do the most damage (kills) as possible. though sniper does a hell of a lot of psychological damage, id rather the enemy see my eyes when i squeeze the trigger.
I'd pick sniper, simply because I wouldn't have to get up close and dirty with anybody (hopefully).
But then again, I would never join the army because I don't believe in violence.
Yes, yes, I know that sounds stupid coming from a guy who plays war games just about every day, but there is a big difference in shooting a person IRL and clicking your mouse to blow a computer generated model's head off.