Of all the specialities FP is the one in which an MPH can be a big positive. It depends though. Whether you have to do a practicum or a thesis for the MPH. A thesis research project related to FP can be a big positive. Sime applies to the practicum. Then to which university you will go? Big names like Hopkins, Harvard, Yale etc will have a bigger impact and will make an impression.
But the big question is that... if MPH is being solely done for the purpose of residency and the application is deficient in other areas such as low usmle scores, no usce, greater than 5 yrs since graduation... I do not see how can a MPH trump them. Though if you have all these credentials and strong lor's, then a MPH can help you get into a competetive program even.
Your best bet will be to ask any program directors or faculties in uni programs who have a MPH degree. I still have not find a definite answer to this. I have my admission into JH MPH for next year. For the 2011 match i will be 2.5 yrs since graduation and with not many gaps in my cv since then. If i get good board scores... Will i do a 3 month usce and do my MPH and apply in the match as a Hopkins grad or should i do 6-8 months of usce and take a voluntarily research position and apply for the match?
The latter will save me a fortune. This i am only going to decide in March next year when i am done with my boards.