As I understand it:
Air Force HPSP can be quite competitive, Army and Navy HPSP are considerably less competitive. And Navy does'nt always fill all of thier HPSPs, so some scholarships go unused. Getting into med school and passing physical requirements are the hardest part of HPSP.
USUHS (military med school)is as competitive as a typical med school. avg MCAT ~ 27-30, avg GPA ~3.5ish. Acceptence rate ~ 15.5%. Though ~50% of the 167 spots are held by folks with prior military service, so some preference may be given to people with military experience.