It depends on the major. Intro classes to popular majors or prerequisites to popular majors/professional programs such as medicine will typically enroll hundreds of students. This is not the case with Michigan only. Most large universities, including BU, will have large classes. Classes in less popular subjects will be small. Class size at Michigan and BU is never a concern. Students almost always receive the help they need, which explains their high freshmen retention rates (97% in the case of Michigan and 93% in the case of BU) and graduation rates (91% in the case of Michigan and 85% in the case of BU).
As others have suggested, both universities are excellent. But unless BU gives you generous aid/merit money, I don’t see how you can justify going there over Michigan at in-state tuition fees.