Well, as far as the two are concerned, Tufts is definitely tougher to get into. If you consider Tufts a "backup", then more power to you.
If you are more interested in clinical "stuff" then Tufts is your school.
If you are more interested in research "stuff" then BU may have the slight edge.
If I had the option between the two, I would go to Tufts. This is not to say that Im not looking forward to going to BU, but I would choose Tufts over most of the schools in the country.