I don't think you can necessarily say that any one school is "better" for primary care than any other. However, UMass as a state school with lower tuition may prepare you better for a life in primary care simply because the tuition is low and loans are less.
Talking about which schools are better, higher ranked, etc, is pretty much a zero sum game. Everyone who finishes med school generally does so by going to only one school, thus their impressions of this one are strongest. Most will feel that that school prepared them in a good way for the future. We also may have impressions of other schools because of some away rotations or interacting with other students, but still the full impression can only be gained by attending a school from preclinical to clinical years.
As a pending UMass grad, which I think Super Rob also knows, I have very little negative to say about the school, especially for those who want a career in primary care. There isn't any real special "emphasis" on primary care during the first couple of years. There are classes on how to interview, you are set up with an outpatient office where you spend several hours a week starting first year, there are lots of opportunities for primary care projects. But a lot of that is true at other schools. As with almost everything, opportunities are found where you look for them or create them for yourselves. The "top" medical schools will do no better at preparing you for a specialty career or a primary care career if you don't take advantage of the opportunities they provide. UMass, for one, has many dedicated primary care clinicians on staff and has one of the oldest family practice residency programs combining rural and city practice. There is also a dedicated away rotations office so you can spend time in another country, underserved areas, whatever. However, you can also have very little to do with this if you want, and set yourself up for a career in orthopedics. UMass does have a very collegial and cooperative environment. Students work well together, attendings respect the students and work at teaching them. I have a few acquaintances at Boston med schools (all 3) and can honestly say that the teaching we got at UMass, when compared with their descriptions of what they went through, seems superior in regards to support and respect for students, learning clinical skills and judgment, and most importantly, being an individual and learning to be "your own" type of doctor. The Boston schools, conversely, do seem to provide a bit more in terms of research opportunities, support for a specialty career, and lectures from famous people, although being someone who wasn't that interested in playing the game of "can you top this" on a CV, for all I know UMass could have given this out too.
And, as I've said before, being a "top" research school doesn't mean much for med students. Every med school worth its salt is going to have a significant amount of research going on. Research advances come out of everywhere. Just going to an establishment location is not necessarily more likely to get you involved with cutting edge research.
Now, one previous poster says go to Rochester because they know people who went there and like it. And I say go to UMass if you can get in because of my experience there. Others will disagree. You have to decide for yourself. Once you have made your decision, be happy with it and make the best of it. When I was trying to find where I wanted to go for residency I went to a few different areas of the country, eventually settled on Michigan over going to Boston or Baltimore, even though I probably could have gone there, because of various factors, not the least of which is that Michigan has a top notch path program.
I just hate to see people make decisions on what med school to go to based on subjective rankings and hearsay. It's always beneficial to hear about someone's experiences with a certain med school, because it raises questions that you may not have heard before or issues you hadn't thought about. But remember, everyone is an individual and is going to develop different impressions. When I was applying to med schools, you could not have paid me enough to go to Harvard med, although if I had ended up going there I probably would have been happy and done well, etc.