Hey man. I first want to say that I do not claim any kind of definitive opinion on the matter, as I am not a physician (yet!) and obviously am not a residency admissions director. However, I've been in Boston for several years, have spent time at New England Medical Center (Tufts' main hospital), and currently work at Mass General. So this is what I know.
Basically, having spent a lot of time at Harvard Med in the various hospitals, I can assure that Tufts is VERY highly regarded around here. A lot of the Harvard faculty I work with at Mass General have strong collaborations with NEMC physicians, and several research projects I'm aware of are conducted at both Mass General (or Brigham) and at NEMC.
Moreover, NEMC, the place where Tufts students spend a lot of their time, is gourgeous. It's a beautiful, very modern facility in Chinatown, which is actually pretty darn cool area. I've definitely seen Tufts students at Beth Israel Deaconness (Harvard's), but not at Mass General or Brigham. Of course, just because I haven't seen them at the other 2 doesn't mean they're not rotating through there as well, I just have never met any.
As for the medical education at Tufts, trust me, it's supposed to be very good. Several Harvard students I've talked to had a lot of positive things to say about Tufts, sometimes bordering on curriculum jealousy, since the Tufts curriculum is less individual-learning-based (err, you know what I mean).
As for BU, all I can tell you is that I've never heard anyone talk about it. I think most people aren't even really aware of it. I know it's quite hard to get into, but the hospital/med center is in a pretty rough part of town. I've also heard that BU students aren't the happiest bunch. Vague, I know, but that's all I can tell you.
So for what it's worth, if you have a choice between Tufts and BU, the Tufts experience is almost certainly superior and Tufts name carries a lot of weight in the medical world. No joke. Of course, BU's no slouch either. Just maybe not as widely known, for what it's worth. You can do well at either place and go very far, no doubt about it.
Good luck in choosing a school.