I did my 20 and if there was a time warp I would do it again. 
Only better ..I'd start in high school and ask the girl out! -grin-
Can only imagine if I had a do-over knowing what I know now.
As for what service is best ..well .. my dad was in Patton's 3rd
..he told me flat out if I enlisted in the Army he would kill me.
I beleived him altho I was kinda enamored about goin into Armor (they offered 1500 dollar sign on just for that back then)
So I flipped a coin, USAF or Navy .. USAF won.
Then talked to some guys who were in ..they told me of the 6-year enlistment, guaranteed career field sign on.
Had no idea about a career but the recruiter said electronic warfare systems tech,
if I passed the tests,
would teach me how to put speakers in my car.
That was the only question I asked him ..LOL.
I passed the tests.
-grin-
The six year enlistment meant I was a two striper out of basic and an NCO (3 stripes) a year later.
That helped a LOT since I was married with a daughter when I enlisted.
Still ..it would have been better to have gone in as officer material ..far better standard of living.
IMHO.
-Frank aka GE