Just keep in mind that there are plenty of research opportunities availabe in medical school as well (maybe even more than as a Master's student), and if you decide that research is what you'd like to do, there are TONS of medical residency programs that are research-oriented, and the program directors will love you to death. An MD is a great thing to have for both research and patient-oriented work.
My thesis MS in Microbiology, Cell, and Molecular Biology, on the other hand, turned out to be a complete and utter WASTE OF TIME AND MONEY. It made me under-qualified for Master's level jobs (which usually require additional experience and/or a management background) and over-qualified for Bachelor's level jobs (which I couldn't afford to take anyway because grad school put me further in the red). Either way, the MS did not help my education. What DID help was being a freshman chemistry TA during grad school because it probably boosted my MCAT a little.
If you get stuck in a supposed 2-year Masters that turns into 3 or becomes completely unsatisfying (been there, done that), then your enthusiasm for research will quickly deteriorate as you idle by. Then you get flack from your PI for "selling out" and wanting to go the med school route.
My advice would be to go straight into med school. Clear a 30 on the MCAT, and your undergrad school won't matter in the least as far as your state med school(s) are concerned. You'll be good to go.