Another route would be to pursue a "traditional" MS at a school where you will be able to really stand out in your program, and get to know a significant chunk of the faculty who also teach medical students/have input into medical school admissions. While I didn't realize it at the time, just by being very proactive in a small-ish basic science MS program attached to a well-known private medical school, I was able to develop meaningful contacts with faculty members who taught in both the graduate and medical schools. It wasn't difficult to stand out because the program was relatively small, and I was studying subjects that I found very interesting and stimulating (and I'm also kinda loud). I knew several students who were concurrently enrolled in this school's (also well-known) SMP program - most of them were miserable due to the level of competition, and did not enjoy the year.
Instead of stressing my way through my MS, I was able to thoroughly enjoy and learn a lot from the courses I took. While it was definitely challenging, it was possible to maintain a high GPA and also "have a life," which is a bit more difficult, it seems, with SMP. Also, because I took all of the first-year PhD-track courses in addition to a couple of medical school courses, had I decided to pursue PhD instead of MD, it would have been an easy transition with no time lost. Added to that was the fact that we had frequent seminars/meetings/talks with the folks from NIAID, DHS, DHHS, USAMRIID, CDC, DOD, etc., and the career-building connections and recruitment efforts left me with oodles of massively awesome alternative options, should I have decided to forgo/not gained admission to medical school. Hell, those options will likely still be open once I graduate from medical school. I would have had no such options had I opted for SMP instead.
Anyway, my point is that, while I'm sure a SMP is very effective at getting you in to medical school, I believe that a "traditional" MS can be just as effective, if you play your cards right - and I firmly believe that it would be much more enriching than simply "acting out" the first year of medical school in a SMP. The trick is to pick a program where you can truly stand out and become known by faculty who can then write letters, talk to committee members, and otherwise vouch for you as a candidate at their school (and likely others). The easiest way to do this is to choose a field in which you have genuine interest.
The first time I applied to the school I am referencing, I wasn't even offered an interview. After completing my MS, however, I received both an interview and an acceptance at this particular school, as well as a few others. The only changes to my application, aside from a new personal statement, was my MS (with my shiny brand-new graduate GPA to stand in stark contrast to my mediocre UG performance) and some great recommendations from the aforementioned faculty.