Re: your title; it's actually called, "HPSP," which stands for Health Professions Scholarship Program, not, "HSPS". However, the important thing to note is that while you are getting your tuition paid and receiving a stipend, you owe service time in return. It's an 8 year contract with typically a 4-year active duty commitment, assuming you received the scholarship for your 4 years of medical school. The rest of the time is served in inactive ready reserve. Again, you will be in the military if you do this program. In that regard, the conventional wisdom is to advise against doing the HPSP if it's just a financial move for you.
I do not believe that the military will directly force you to do a residency in a given field. You apply for the military match, which is similar, in many respects, to the civilian match. However, your choices will be more limited, obviously, as there are going to be far fewer programs to choose from as compared to the civilian match. Also, there are features weighted into the military match that are not in the civilian one, such as prior service time, which may impact your ability to match. The military has its priorities with respect to medical fields, therefore the number of residency slots per field or program may be dependent on what is needed at the time. I would say there typically tends to be a greater number of FM, IM, and GS spots than other fields, but this can certainly vary and may be dependent on branch. Good research is necessary when applying. One important difference between branches is that, in general, Army has the most programs. The USAF has fewer programs, so historically, you would have a higher probability of doing a residency as civilian deferred versus the other branches.
Another factor to consider is the possibility of being assigned to serve as a GMO. Basically, this amounts to working as a GP and results in discontinuous residency training. You complete your internship year and then serve as a general medical officer for a given period of time and then complete a residency program afterward. Historically, you would have a greater chance of being assigned to serve as a GMO in the Navy. There are some good aspects to being a GMO. One is that when you complete a GMO tour, you may have an easier time applying to a given program as a PGY-2, due to accumulated active duty service time. One negative is that you have discontinuous training and it will take longer to complete. Some people actively try to do GMO tours so that they can burn off their commitment.
Anyway, based on all the above factors and more that I didn't mention, it is possible that you may be indirectly positioned into a field that you did not originally plan on. You do have choice, but you also have to remember that the needs of the military come first. Please also keep in mind that while it seems like you are better off coming out debt-free, you may also get paid less as a military physician. There are plenty of financial analysis posts on SDN to that regard and most of them show that unless you do primary care in the civilian world, you aren't going to be at a financial advantage, even with no debt. The HPSP program is NOT a free lunch. It is designed to make military physicians. To reiterate the conventional wisdom: don't do the HPSP program unless want to be in the military.
