If you are interested in doing a MD/MPH, then I would suggest that you enroll in a MPH program that is as quantitative as possible. (You mentioned in your post biostatistics and epidemiology -- that's good.) The MPH degree can be a good thing to get, and it can also be an expensive waste of time.
Good reason to do it during medical school: You can use the MPH to build up your foundation in methods (biostatistics). Hopefully, while you're doing your MPH you can do some research assistant work with one of the faculty.
Disadvantage of doing it during medical school: You don't really have any clinical experience to speak of, which makes it difficult to focus. Fortunately, you don't really have to focus yet. Just focus on putting together a skill set.
Good reason to do it after medical school (say, after residency or as part of residency): If you combine it with residency (say, doing a residency in preventive medicine), then you can draw a salary while getting a MPH.
Disadvantage of doing it during medical school: If you haven't nailed down the methods by now, you will have to spend most of your MPH time learning methods. And if you want to launch into a research career, you would probably do a K-08 or a post-doctoral fellowship to narrow down your research agenda. (If you do the MPH during medical school, then your postdoctoral fellowship will be much more focused and probably more productive.)
What you don't want to do (and I wholeheartedly admit this reflects my biases) is do an MPH and do a concentration in something like "management" (unless there are MBA-ish elements to it) or "health policy" or "international health" (unless there are really important cultural/travel/research elements to it). If you do biostatistics/epidemiology, you will likely waste your time the least (since you can always take those tools and apply them to any clinical setting in the future, regardless of what clinical direction you choose).
Cheers
-a.
