From my experience... Go Air Force if you want a high quality of life while serving in the military. Go Navy if you want pretty much the same thing as the Air Force but don't mind being on a boat for months at a time. Go Marines if you have a massive sense of pride in yourself, your country, and your brothers in arms. Go Army if you want a signing bonus(Honestly...). Go Coast Guard(Lol...) if you don't want people to know what you do(other than protect the coast).
There's no real difference between the branches besides their heritage and how members treat each other. People might say you need to be sooooo physically fit to join either the marines or the army, but that's simply not the case. Check their PT standards for basic training. I'm pretty sure the air force has stricter standards than the marines(Besides the fact that the marines actually have to do pull ups). It really depends on what you're willing to put up with.
As for the post 9/11, I'm pretty sure it's 36 months.
If you are genuinely considering joining up and want some information, shoot me a PM or whatever. I am personally in the Air Force, but I was stationed on an Army base with a very large Navy presence. Your choice though.
