i think you can do ROTC during your time as a premed major, you'll need to takl to an AMEDD (for the army, not sure wht the other branches call it) recruiter for specifics.
or you can talk to one after getting your MD about joining.
as for which branch is best, it depends on what you want to do.
if you would like to be a doctor for a line unit, where in the states you'll run a BAS thats for your battalian only and where you'll be the only doctor there...and run an aid station on deployment (most likely with another doctor or two) on a smaller base where there is less of the dog-and-pony-show bullshit going on: army. then again you might get stuck in a hospitol, its all luck of hte draw. if you go to the line though you'll get THE MOST experience with trauma patients though.
if you want the best possible life style with the least deployments: air force
if you want to work on a ship, obviously navy.