Both are really strong programs, and more than anything it comes down to gut feeling from each school. Personally, when I was interviewing at both I felt that Tufts students were happier and more laid back, but that is obviously a personal opinion. Objectively speaking, the BU curriculum is a bit different, and IMO more stressful in that you start with anatomy right away and do at the same time as basic sciences (at least the last time I checked) whereas Tufts has a more gradual ramp up in difficulty. BU has a great new housing option, but Tufts has a much newer library and study area, so there is that to consider.
If you are from the area and are really torn, definitely take advantage of second look days and/or contact both schools and ask if you can go back and visit again. Do both in the same day to get a good comparison of both the students and physical space
While I prefer Tufts for the reasons stated, BU is clearly superior in terms of prestige/ranking/training however you define it.