Do a search in this forum...
HPSP: Because you want to be in the military
USUHS: Because you want to be in the military for longer.
Any other reason, fuhgeddaboutit!
USUHS can also give a small advantage when you apply for residency in the military GME program as you will have rotated through many of your services programs as opposed to the HPSP people. Have to wear uniforms to med school.
That's a popular myth among USUHS students and unfortunately not true.
Have you seen the selection board scoring sheet?
Not true. The myth is that simply being a USUHS student gives you an edge (because it's the "Military" med school). The reality that I have been told by a few program directors at Madigan is "showing" your face to a program by doing a rotation there and performing well during that rotation goes a LONG way. The Rads director at Ft. Sam told me if you don't rotate there they won't even consider you. This is the edge that I am talking about.
HPSP: Intern year + 4 years GMO/FS/DMO -> freedom in 5 years and hopefully some good life and medical experiences
USUHS: Intern year + 2-3 years GMO/FS/DMO + 2-4 years Residency + 4-5 years payback = ~12 years (more if you get suckered into fellowhip). You really can't/shouldn't be a GMO for 7 years.
USUHS is a huge commitment. Unless you have been in the Navy and understand what you are getting into, avoid it. If you honestly want a career in the military, meaning you have prior service (not you talked to a recruiter), it's a fine choice.