BC and BU are in the same group ranking-wise. In terms of level, they’re essentially the same. The big difference is that BC is Catholic and Jesuit. It’s open to all faith traditions and attending mass is not mandatory at all, but it means you need to be comfortable with mandatory Theology classes (more like philosophy of religion than catechism!), crucifixes, and priests or Jesuits around campus.
https://www.bc.edu/bc-web/jesuit-catholic.html
Compared to BU, there’s a definite campus and more “bro” culture.
BU feels more urban, less “jocky”, similar to NYU in some ways (with better housing, reputedly). It’s non denominational.
You should try and read descriptions in the Fiske Guide to tease out the vibe at these different colleges.