Mitch,
Just some general info you might find useful as you make your decision (keep in mind here that I am a little biased...
First, while UMass is one of the top 10 primary care schools, and is well known for producing good primary care physicians (i.e. communicators), it's important to realize that less than 50% of our school goes into primary care each year. Don't let the primary care focus dissuade you if you are interested in specialties. Really, the primary care name comes from the fact that you get early clinical exposure (working in clinics within your first month of school, interviewing and examining SPs, etc.).
Academically, we consistently score above the national mean for both USMLEs and match very competitively... Another example: the mean Surgery SHELF exam score for my class this fall was the 77th percentile nationally.
Second... as far as money is concerned, there is really no comparison. Thanks to the friendly people in the legislature, and the < $500 rent in Worcester, it is a bit more economical. And if you love Boston, you can drive there on any given night... In fact, you can live there. About 10% of my class actually commutes from Boston.
Research... well, this is tricky. UMass has the second highest research budget of all MA medical schools (you can guess #1), and the labs are growing. However, most PI's are not as well known as those at BU... If you want to do serious research or want to get a PhD, BU may be the better option.
Lastly, you need to get a feel for the student body and attitude... I would speak to some BU students as well as UMass... my overall feel based on discussions with friends is that BU students are more academically driven and a little more competitive, UMass students are a bit more laidback and cooperative. I think that there are advantages to both, and you should go where you fit best.
Hope this helps... to echo skypilot, both schools are great. I would choose UMass, if for no other reason, education is at the very least equal while the price is not, giving you a bit more flexibility for the rest of your life.
PS Whatever you decide, make sure you let UMass know right away... for as long as you hold your position, we cannot offer it to another student (they probably talked about this on your tour day).