it depends on your reasons for getting a masters. An MPH is a good supplement to an MD degree, but it does not replace a lacklustre undergrad GPA. if you are looking for something to impress adcoms, or show them you can handle the course load, the masters in science is definately the way to go. Why? because although public health is related to medicine and is quite an interesting feild of study, the course load is not as challenging as a science masters. On the other hand if you are simply looking for something to do during your lag year (assuming all other aspects of your application are solid), the mph might not be a bad idea. it all depends on your motives. Hope this helps.