Yeah, I've been told the same thing, not to underestimate the 4 vs. 7 years of commitment. That 3-year delta could be a significant time-period in your life. So this should probably be your main decision criteria, along w/ your motivation in becoming career military (which, incidently, some USUHS grads don't do, they just get out, even at the 13- 14-year mark).
So if your set on doing a mil res, then this shouldn't weigh into your decision, b/c you'll go this route either way. If anything, USUHS would probably be the better route, b/c their grads (I think) get a better look for the mil match.
Truthfully, I think you'll find camraderie at most medical schools. It's not like college where you have a bunch of cut-throat pre-meds running around. Of course, there'll always be the competitive bunch, but I think for the most part med students are more cooperative and jovial with e/o. All the schools i visited, everybody seemed to boast their sense of camraderie. So I wouldn't weight this too much into your decision. Would instead focus more on the career long-term aspects of things, b/c that's really the difference between USUHS and other schools. When you go to USUHS, you're not just going to a med school, you're making a commitment to something for a significant part of your life, possibly 20 years.
Bottom line advice I can give is if your weary or doubtful about it, probably better to do HPSP (shorter commitment). It's nice that you have that option. If you're gungho and want to do the military for a longer period of time, the clear better option is USUHS.