There was definitely no difference in GMO tour assignment with USUHS vs HPSP in my internship class. I think USUHS may have had a slight advantage in getting accepted to DMO training because of people they had met along the way and the ability to do rotations with DMOs in interesting places to "show interest" as medical students. I am a product of a 4 year HPSP, and played the game right in early internship by getting to know the appropriate people, doing a 2 week DMO elective, etc. to secure a spot. When our billet list came out, it made no difference where we went to med-school.
Remeber, USUHS grads must take a military internship, while HPSP have the option for a civilian deferment. I would recommend the military internship, but I had a DMO classmate who came from a civilian OB-GYN internship. Also remember, just because someone is from USUHS doesn't mean they'll automatically suck up one of the desirable internship spots. They have to do a military internship, which means some (not many) can end up in places no one else wants to be (FP, Psych, etc.). HPSP can always go out to civilian for a PGY 1 surgery spot, etc. if the Navy doesn't have room. Most people I know got their first or second choice for internship (location or specialty). Transitional, Surgery and Peds internships are the hardest to get, Psych and IM are the easiest.
DD