If you really want a master's degree, then get one. If you really want to get into med school, take upper-level undergrad science courses. These are the classes that will actually raise the GPA that adcoms care about. If you've never taken the med school prerequisites before, consider a structured Post-Bac program.
Otherwise, if you're like me and you've already taken the med school prerequisites but got B's and C's, then you should take upper level (3xx and 4xx) biology, chemistry, physics, and math classes. You can do this on your own at your own pace at your local university. It will be cheaper and you can withdraw any time you need to if you get in.