i joined the air force, as i saw it as the best deal for me. i went in with a guaranteed job (ie. it was in my contract that i would get the job i requested).

just because you learn a skill in/for the military doesnt mean that it is likely to not have any real world application. many jobs can be carried over into the civilian sector. you could become a consultant to companies manufacturing equipment you dealt with every day.

the different branches are just that. i chose the air force as it is more geared towards a higher technical level and takes care of its people a little better. but then, each branch also has different missions. the army and marines are more likely to get shot at than most air force and navy, simply because of the way that combat is heading these days.
im not an expert on any of these subjects, these are just general thoughts and views.

i am happy with the choice that i made. i cant guarantee that you would be. i think that the best advice that i can offer is this : learn everything you can before hand. shop around. talk to the different branches and talk to different recruiters in the branches. if you know what you would like to be doing, try to get a guaranteed slot. if you leave the job placement decision up to the military it is very possible that you may end up doing something that you dont want to/wouldnt be happy with.

note : none of the above comments carry any official weight. they are my viewpoints, nothing else