If you're planning on US military aviation, you must be a US citizen. All pilots are officers, who must be citizens.
All US military branches are essentially comparable, and in fact USN, USMC, and USCG (Sea Services) all use the same training and procedures, and the USAF often mixes students with the Sea Services. Army is a bit different due to emphasis on helos, but still equivalent quality. The real difference between services is more the type of flying and location of bases.
All trainees complete the full program regardless of prior flying experience (possibly one exception for commercial pilots going the guard/reserves?).
I think most of the US services now have applicants do a little general aviation to assess their aptitude, but this is not really training as much as assessment.