It really depends what kind of a school you want. I'm obviously biased towards Tufts since I chose it. Are you looking for a clinical school or a science/medical school? If you're looking for a clinical school, go to Tufts. If you're looking for a more medically oriented curriculum, go to Columbia.
I was also accepted at both Tufts and Columbia and I chose Tufts because I wanted to go to a clinical school. For example, we're taking Operative right now in 1st year. At Columbia, you don't do it until 2nd year. Also, the students here seemed a bit happier-I remember many Columbia students complaining about all the hoops they had to jump through. Not that it isn't tough here too.
I have to strongly disagree with one statement that someone at Columbia made. I remember she did our tour and said, "OK, the cat's out of the bag. Anybody can drill-a 13-year-old with some manual dexterity can drill. Here, we teach you to be a doctor." She was doing that to explain why Columbia emphasizes science so strongly. After months of Operative, I must disagree-drilling is hard!!!!
Hope that helped, Mike