I went into the Navy for a job that would give me a career experience and college credits in the area I wanted to perform as a civilian. If I hadnt of known what I wanted to do and just wanted to get into the military in general, I would have chose Air Force.
So make sure you consider what you want to do after the military because even if you stay in all the way to retirement, you might still be pretty young comparitively when you get out and still want to work for a while especially if you get out with a college degree and other skills that make you marketable in civilian sector.
The years go by faster than you think. I know it might be hard to pick the right thing if you dont truly know what you want to do, just DONT go "undesignated" (no specific job code) choose Some Kind of job and get your military schools out of the way when your young because it sure as heck doesnt get easier to do all that physical conditioning the old you get.
NRA Endowment Member