Congrats!
To begin with, I don't think you can do wrong with either school in terms of quality of academics, so it really comes down to how you felt about each one. Was there anything about either that really stuck in your head on interview day? Did you have an instinctual sense that you belonged at one more than another? It is a very personal choice, so first try and get a sense of if you would feel more comfortable at one over the other.
That being said, I will say that my experience at Tufts has been fantastic. The program of classes is good at keeping the level of work manageable, and there is a lot of opportunity for clinical exposure early on. One of the things I find most important, and yet very hard to see on paper, is how well the class meshes together. Admissions does a great job of picking a group of people that fits well together, but has a great deal of diversity in terms of interests and experience. Plus, being in the middle of Boston is a lot of fun.
If I had to make the argument for UMASS, it would be that it is far less expensive for a quality education. The difference in tuition (20-30k I think) is significant. The importance of this depends on you though. If you are planning on starting a family, or have lots of past debt, or have any other financial issue, then the the cost difference is extremely relevant and likely will be the deciding factor. If, on the other hand, money is not an issue, then cost is not as pressing. I can't speak to the specifics of Umass' program, but I have never really heard anything bad about it except for being a bit isolated.
If you have any questions about Tufts, feel free to PM me. Also, don;t be afraid to call the school and ask to visit if you need another look at the place.
