I am also deciding between BU and UMass, and I'll say this. I have been continually impressed with UMass and will likely attend there. Reason #1 is the money. Cost of living in worcester is much cheaper, I can have a car since there is plenty of parking, and I save a boatload on tuition. I will be entering residency with no debt. Further, UMass is actually outranking BU in NIH funding as there has been a HUGE recent push by UMass to become a top research school (they want accomplish that by 2009). Hence, you have the incredible new, state of the art research building, and the new trauma center that will truly allow UMass med to earn the level 1 status. The new construction as I understand will be done by the time we enter and it's going to look great and modern. The student faciliities (i.e., lounges, lecture halls, study rooms, etc) were actually better than those at BU. Plus, at BU, you've got all those other students in other programs in your classes.
I actually love BU's location. It's hospital is very nice and the campus has a pretty lawn and lots of other amenities that UMass doesn't have. But as far as quality of school goes, I think BU would be better socially, but it's not worth the money academically.
One thing about residency matches. UMass is an all around good school and is being recognized as so. you'll see it's rankings go up very soon (next few years) because of all the new research dollars they've been getting. They've opened up a new cancer research center and are doing a clinical trial of an AIDS vaccine. BU is getting the BioPark on its campus, which is pretty cool, but that's more for private companies and I don't see that translating into dollars for BU.
Residency match is about the individual more than the school. Yes, the school is important, but it's how hard you work and what you want. UMass won't shove primary care down your throat. In fact, it's primary care focus is good because you get clinical experience right away in your first year. UMass people have a great reputation for being good clinicians overall. So, don't let the primary care thing scare you. It scared me until I did more research about it. If you don't want primary care, they have all the resources and the ability to match you anywhere. Lastly, you can plan your rotations in boston if you want and get residencies there. I honestly don't think there is an appreciable difference between the two schools (BU and UMass) as far as residency directors are concerned. Maybe someone can back me up on this.