You asked it this affects your chances of getting into med school. The courses you take affect your GPA much since when you apply you'll only have a semesters worth of grades to report (most won't count as science). What it does affect is your understanding of the health care system, your knowledge of reseach theory and methods, and added practical/working experience. I think having a MPH is no better than the gajillion of other things you can do on your year or two "off".
People take many different routes (research, pharm co, travel, business, post bac, teaching, etc.) before going to med school, you can make most anything sound like it will make you be a better physician. SO pick what you want to do the most b/c YOU want it, not because you think it will make you a better candidate. If you do what you love, you can't go wrong.