I have noticed that there are a lot of people on this forum that had some pretty bad experiences in the Armed Forces..I think it depends on the attitude you bring. HPSP is a great way to get your degree with no debt. Now, as with most things in life, nothing comes for free. You will have to give the Armed Forces time for the monies they spent on you. If you look at this as an opportunity to do something you would never have the opportunity to do then you will have some fun, see some amazing things, and, most likely, put off your medical education for a bit. GMO tours are notoriously void of any medical growth and can last from 1 yr to 3 yrs. You deal with a lot of administrative BS that has nothing to do with your medical education. It does teach you something about dealing with people and, hopefully, about leadership.
I did have some bad times with horrible Commanding Officers but overall I got to fly (yes, at the controls) 100ft over the rivers in the Phillipine Islands and Micronesia, scuba dive in Palau and Oki, shop in Osan, fly a Phrog over Iraq, help injured Marines make it home from Iraq, and many other things that my contemporaries would never see or do. I also had the honor of meeting some of the greatest people I will ever know.
Now as for which is best....all my experience is with the Navy but from my view I would agree the Army seems to have the worst billets. The Navy has opportunities to fly (what I chose), dive, or be an MD on a ship or shore. The Air Force has the most money but seems to be the most bureaucratic and frustrating, depends on what you feel your personality can handle. I chose the Navy because most of the bases where in decent locations...I figured if I ended up on a ship at least Iwould be near the ocean
Going straight through to residency will never be guaranteed. It depends on the needs of the Navy/AF/Army. If they need that particular specialty and there is not enough qualified folks coming back from GMO to fill the residency spots your chances are good. Do not listen to any promises your recruiter says unless it is in black and white.
The other tidbit they don't tell you is that if you serve your time and it is less than 8 years then you are put into the IRR (Inactive Ready Reserve). This is a pool of personnel that they can call back if they need them. In the past it was unheard of to occur - I have seen it happen recently. So be aware of that.
The way I looked at HPSP was the worst case scenario....I finished internship and was put on some small ship out of Norfolk which spent 7 out of every 12 months at sea...could I make it through the two years? My answer was yes, I could make the best of it. Think long and hard for once you sign on the dotted line you belong to that organization. If you come in with the right attitude and take it all in as an experience which has an end point and can offer you some personal growth you will do fine.
Good luck with your decision!