MPH is a great idea if you want to pursue a career where it would be useful. e.g. if you want to go into academics, it would be helpful. On the other hand, most good universities would offer it to you as a part of your fellowship, all expenses paid. You have to balance that against taking time off while you are still a med student.

It can help you now to get into a better residency program and a better fellowship. But if you think you can get into good programs even wothout that, it would be advisable to defer it till your fellowship.

If you want more specific advice, dont hesitate to email me. I did my Masters when I was on faculty, and it helped me tremendously in the work I do.