speaking as someone who did the MPH before med school i'd say that it was an excellent experience that provided many opportunities and a great knowledge base for reading literature and doing high end research and statistical analysis...
however, i would advise you be wary of the combined degrees, especially at schools that do not have a designated school of public health (like downstate) ...the programs tend to be thrown together and taught by MDs who just happen to have picked up an MPH along the way instead of PhDs who are the leaders of their field (at least that's how it is at another SUNY that shall remain nameless
). I did my MPH at a top 10 public health school and it was an excellent experience inside and outside of the classroom and hands down the best two years of my life.
If you have a real interest in the degree I would suggest you hold off on it until residency (or beyond) and if you still want it you can do a 1 year program at one of the top public health schools but trying to cram it in at downstate would, in my view, be a mistake since it will add more work to an already overflowing schedule and will rob u of your summer when you should be regaining your sanity, relaxing, and building your resume without giving you enough of a return on your investment to make it worthwhile.