Class rank and board scores are the 2 most important factors when applying to specialty programs (and, I suppose, GPR programs).
If you're looking to go straight into private practice, personality and clinical skills/confidence trump your rank n board scores. A hiring dentist could care less about how well you can recite the spinothalamic tract; he/she cares more about how likable you are and how much money you are going to make for their practice.
I'm not suggesting that slacking and being last in your class won't hurt you, but middle of the pack ain't a bad place to be.
As far as which school will prepare you better for the boards, no one can tell you. I imagine Penn boasts higher board scores, but is that because they are really better at preparing students or just that highly motivated students attend (wow, I sound like armor 😳)?
As far as what school will make you marketable.. it really depends on the dentist doing the hiring. If the Doc is a Tufts grad, they may or may not be inclined to hire you b/c you went to tufts, or vice versa. Bottom line- neither school will make you significantly more attractive than the other.
jb!🙂