The debt load as a student is something that you need to ask your school rather than us since it is completely school and city dependent. Obviously the cheaper your school the worse the decision HPSP is from a financial standpoint.
Navy HPSP has a lot of advantages and disadvantages but the biggest thing you need to understand is that once you're in
1) You're stuck. From the second you get your first check there is no getting out until you've served your commitment no matter how much ou want to.
2) They own you and can change almost any rule on you. Just because most people now only move once every 2 years it doesn't mean they can't move you every 3 months. For that matter they're free to send you to Iraq for your entire commitment if they want. This is an oranization where not listening to your superiors is actually a crime, so if you plan on being a Dr. Cox type you might want to look elsewhere.
3) You need to do your residency with the military unless they give you permission not to, and they will almost never give you that permission. First the military match has different odds than the civilian match. For EM in particular the military mach is way harder, Ortho on the other hand is a little easier. Also be aware that the military match works like the civilian match did over a generation ago: you apply for an intern year and then, when you're done with THAT you apply for a residency. This sounds reasonable except for the fact that a lot of Navy HPSP students (more than half) don't get a residency, at least not on their first try. They instead get to spend 2-4 years as a practcing physician in the military with only an intern year as training, known as a GMO tour. These tours are generally cool and are often a lot more specifically 'military' than what you'll do after you finish residency but it's also 2-4 years where you dont learn much and don't get paid much and your skills rot and it's an extra 4 years before you have the stability to actually stop moving every year or two and start a family
I'm an HPSP student and so far I feel like I made the right decision. Financially I go to one of the nation's most expensive schools, I'm interested in a low paying specialty (Psych), and honestly without the military I think I would actually have needed to do something I liked a lot less because there's no way I could have paid off my debts as a psychiatrist. It's also nice that military residencies pay twice as much so I may be able to buy a real house and start a real life before I finish that. Finally, and most importantly, I've always had a thing for the military and so far my few weeks of contact with the Navy have been the shinging spots of medical education. The residents and attendings are infinitely nicer and more relateable than anyone in any civilian hospital I've rotated at and it's a lot easier to stay motivated when you know that your patients are heros (or, at the worst, an E-2 trying to get out of a PFT) rather than addicts trying to scam narcs off of you. That being said I suppose it's easy to feel satisfied when I haven't made any sacrifices yet and I may change my tune after my first stay in a warzone or the first time the military mindlessly and needlessly F--ks my career.
My advice: don't do it if your school is cheap because there will be better opportunities to join later if you're not taking on a lot of debt. Also don't join unless you at least like the idea of a military life. I'm not saying you should have always fantasized about joining the Marines (though I did) but if you're the kind of person who considers Vietnam veterans war criminals and thinks of Soldiers as idiots then find someone else to work for. Also, plan on joining with the intention of deploying, because once you sign the papers you're not getting out of it.
Further advice: Consider all 3 services and read up on the relevant differences. If possible try to go to basic training before medical school starts. Finally get the ball rolling on your paperwork now as this might take a while and they do occasionally run out of scholarships. No signature counts except the last one on the actual contract, so it's in your best interest to submit your forms and go through medical processing early in case there's a problem to iron out.