Ok, this issue is dear to my heart.
For me, I decided between an MPH and a one-year MS program in Physiology (or something similar like Biomedical Sciences, etc). Hard core science has NEVER been my bag - so I was very scared to sign up for one of those Master's programs where you supposedly prove yourself and then you get into med school. For me, I didn't have any serious study habit problems when I took my pre-med classes - I just flat out found them very hard - even though I tried as hard as I can. So its not that I think I couldn't handle an MS Science program - its that I probably wouldn't dominate it.
I'm much more fascinated from a personal/intellectual viewpoint in Public health, outcomes, bioterrorism, healthcare for the underserved - and consequently I'll do much better in these classes because they genuinely interest me. I agree with Adcadet - an MPH degree can be as marketable as you choose to make it. If you want you can turn an MPH into an amazing experience that would clearly set you apart from Joe Applicant. Or you could just add it to your AMCAS application and not say much about it - in which case it probably won't really help you too much. As far as getting into the school where you are getting the MPH - that probably depends on the school. But the way I see it - if you are applying to X School of Medicine, it can't possibly hurt to have a few recommendations that are written on X's letter head.
So, I would say that if your end goal is simply getting into medical school, and you are sure you can not only handle hard core medical school level sciences, but actually do WELL in these classes - then I'd do a one year MS in Physiology (either Gtown, Finch, BU, etc.) The bottom line is that there is considerable data out there that supports the fact that an MS in Physio helps you get in. Thats what these programs are designed for.
However, if you are like me, and you are more interested in public health, and if you view the first two years of med school as a necessary evil en route to being a doctor, then I'd do an MPH - you'll be happier, and you'll therefore do better.
And Dartmouth has a nine-month MPH program
http://www.dartmouth.edu/~cecs/educational_programs.html
that would allow you to enter med school right after - but you won't find it on the ASPH website since this is the first year of the program and it therefore won't be accredited until June when the first class graduates. Good luck.