I am not a doctor yet, but here is my 2cents:
I have a similar choice and am going with BU (or Tufts) because I liked the facilities and the location better. Also there is more research opportunity at BU. I am buying a house in Boston. And Boston Medical is awsome!!! Also, the matches are not comparable at all. I liked BU's matches much better, less PC than UMass. Most importantly, money is not a factor for me since I am on full scholarship.
Having said that,
I think that you should choose Umass for the money. If you ask US News and WW (for what it is worth), Umass is better than Harvard for Primary, and working its way up the research ranks as well. Taken for all in all, Umass is about the same, possibly a little better over all, than BU.
UMass is working very hard to increase its reputation and its facilities, that can only mean good things for students.
And I agree with the cocky bastard up top (Doc05) who says that there is nothing about BU that would justify paying 15,000 more. Worchester really sucks, but it is much, much , much, much, much, much, much, much cheaper to live there than in boston. So your savings will not only be in tuition, but in rent. Potentialy 6,000+ per year difference. Also UMass has the 2/3 tuition program. Remember, when you borrow money, you may pay back more than the amount you borrowed becasue interest sucks.
Both schools are equal except for the money. BTW UMass and BU both have EXCELLENT names in the Boston area, according to the people I know at MGH.