I signed the contract soon as I turned 17, I finished highschool and only had a 4 day summer vacation then went to Parris Island. MCMAP (http://en.wikipedia.org/wiki/MCMAP) is some tight stuff.
I learned three important things from the military.
1. Whatever you want make sure you get in writing. Words don't mean squat.
2. Always carry toilet paper, military facilities are notorious for not having it:)
3. You can make a sandwich out of anything, specially in boot.
As far as the Navy and Marine Corps... the Marine Corps is a department of the Navy, the men's department, j/k. I am in the only TRUELY integrated Navy/Marine Corps unit, a medical battalion. Trust me, it gets confusing as the Marine Corps places more responsibility on lower ranks than the Navy. E-3/E-4 in the Corps is when you start seeing some major leadership whereas you're looking at E-5/E-6 in the Navy. Right now there is a toss-up for Platoon Sergeant (a billet not a rank) in my unit and they're looking at either me or a good friend of mine... both of us are Lance Corporals (E-3). I've never hear of anything that big going to an E-3 in the Navy.
As far as Army vs Marines... my best friend is a ranger, currently selected for special forces and I can still kick his ass lol.
Tae Kwon Do - 11 years
MCMAP - 3 years
Aikido - just started
Kendo - just started