If you get an MPH before med school with the intention of boosting you app, it will be a waste. If you get it before med school with the intention of working in public health for a while, then applying to med school, you can get some great experiences if you look for them. Generally though, if you know now that you want both degrees, you should probably get your MD first. I encountered clinicians of all sorts while getting my MPH, and they always seemed to both get more out of and contribute more to the classes.