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.
The Army and Navy have pre-select policies for the HPSP, meaning if you have a certain MCAT score and GPA you will get a spot if one is available. However, I do not know what these scores are at the moment as they have changed in the last year. Your recruiter should know.
As far as I know, the Air Force does not have a policy like this.
GPA 3.78, MCAT 31...I was accepted by HPSP and USUHS...I decided to take the Navy HPSP scholarship to attend a New York City based medical school because of the better education (NYC has crazy PATHOLOGY everywhere!), great location (Clubbing and bar hopping is fun in NYC), and less commitment (4 years vs. 7 years).
I'm currently a Naval Flight Surgeon (aka "flying high GMO").