if you decide to masters route, go for a REAL masters like one in biology or biomedical engineering.
All the teach in a MPH program is that doctors dont desirve any money and the doctors are the root of all the problems in modern medicine.
Only if your undergrad is shady. If your undergrad GPA is solid say 3.7+ and MCAT is poor say 27 or less then MPH won't really help you. But MPH will help you only if there is enough science course work in that program I believe 20+ credit hours of science courses. If you only have 8 credit hours or so of science courses MPH is useless. I mean yea it will give you somehting to talk about at the interview, but real usefulness as in puttting you ahead of someone with 0.5 more in their GPA is unrealistic.
Best way to improve your chances if your GPA is low, go to a post-Bacc program and take many science courses. THat will definetly work. Also another interesting option is to take nursing or PA, that way you have a solid job in your hands when you are done.
But MPH do only if you want to pursue the business route, don't do it just to improve your chances for med.