I am currently a part of the Air Force HPSP. I chose to only apply for the AF HPSP for many reasons.
First, my family has been Air Force in the past.
Second, I worked with many physicians that had been in the military. The Navy doc said he'd choose the Airf Force, the Army doc said he'd do the Air Force, and the Air Force doc said he'd do the Air Force. 3 for 3 agreed that the Air Force was the best.
Finally, the quality of the bases and facilities that I have seen in the Air Force seem to be far better than the other branches. That is just a personal opinion that I have.