if you're contemplating marines..."all marines are riflemen first, everything else is gravy"...they are the redheaded step children of the dept of the navy...if you're wanting to serve, but not fight...go airforce, with the right m.o.s. you could have a lifetime career...navy would be about the same.
Interesting bit about serve but not fight... USAF members have been deploying non-stop for 20 years for AF missions, PLUS serving as augmentation for the Army for things like convoy security and drivers, etc. USAF has been doing both USAF's job AND Army's job since 9/11. Same with the Navy... In addition to their normal cruise cycle, the Navy has also been augmenting the Army for jobs that are delegated to the Army by law (a law that has been waived yearly by the secdef btw).
Join the Army and enjoy watching the USAF and Navy bail you out by doing your job in addition to their own, is more like it.
Don't get me wrong, Army does a great job. But they apparently don't have enough people to do the job so USAF and Navy have been stepping in to fill the shortfalls ever since 9/11. It's a terrible waste of money since every augmentee is already fully trained for their normal job and then have to be retrained from scratch to be able to do the Army jobs they're filling, but that's how it is.
As for which service to join, just look at what they do every day. Army gets down and dirty and does the bulk of the ground fighting and logistics. USAF gets dirty when the job requires it, but 90% of the USAF's job is highly technical and not quite so dirty. Plus the USAF is "normally" operating in support of the Army whenever there are troops on the ground. USAF often only operates on it's own agenda between wars or in the first few weeks of a war before the Army crosses the line. Navy spends a lot of time at sea, go figure
Marines do everything the other services do, but on a smaller scale, with less money, and more attitude.
They're all good choices if you want to serve, but they're also a good way to get killed and the BS factor is pretty high too, so you must REALLY want to serve if you want to have a good experience in the military.