I already had my public health degree before applying to medical school, so I thought I'd give my 2 cents really quick because i think I'm going against the majority (might be 4 cents):
1) I have heard that getting your public health degree after you've done some medical courses gives you a better perspective on the public health degree and the rest of your medical education. This is in contrast to doing it first, where people tell me that I have to be cautious about not losing what I've learned in public health when I'm in medical school for 4 years and trying to integrate it into my education since I did them separately.
2) Cost is obviously a huge factor, you save money doing them combined because they will let you cut off time. Let alone the cost to reapply and fly to interviews, etc.
3) If you are getting into Hopkins now, you can always go get an MD, take a year off between your 3rd and 4th year and go get your degree from Hopkins then. Public Health is a lot less competitive to get into than medical school, so you will probably get in again.
4) On the flip side to that, I think having a public health degree can help you get into more medical schools in the future if you take the time to really distinguish yourself during the next 2 years through research, service, or whatever else. (Not to say Tufts isn't awesome!!!)
Everyone's situation is different. I've been working in public health for a while and have saved up enough money to handle the extra cost associated with doing them separately, so that becomes less of a factor for me, and I am very thankful for the schools I got into, which I think is attributed to what I did during my public health degree/career. Therefore, I chose, and am happy with my decision, to pursue public health first. However, in the end, you gotta do what's right for you. Good luck!!!!!