I would suggest to go to UMass, the chances of getting into MIT are slim for everyone even with great scores. Also, you are a good fit for the program and that's what PhD studies is all about. It depends on what you want to do. You could always get a job as a tech for a year and netwrok at MIT, sounds like your good at that.
On a side note, I also applied to Umass Amherst with a Master's degree in Biology, great numbers, letters and 7 years of research experience but have not heard a thing. Do you know if they are having later interviews. Congrats on your acceptence, if I were in your shoes I would go to Amherst, it's less about the prestige of the school and more about who you work with and what you do, you could publish 4 papers at Umass and only 1 at MIT, or vica versa, who knows. But yay, good luck!