My bias is against UIUC and toward Michigan, but for me this is not even a close question. UIUC is not only debt free but close to zero cost. Absolutely do not borrow to go to Michigan instead.
I would be more sympathetic if the student’s more expensive, preferred option were Amherst or something like that. That’s a completely different experience than Illinois, and different kinds of kids would thrive in those spaces. But one excellent big 10 flagship versus another? No question, avoid the debt.