@Alexandre, the quality of the student body also matters when it comes to university reputation (and affects the quality of the alumni).
There not a big difference these days, but there is a difference, and that shows up in alumni achievements:
http://talk.collegeconfidential.com/college-search-selection/1682986-ivy-equivalents.html
UMichs alums do well, but on a per capita basis, they’re just below the Ivies (while NU is within the range of the Ivies; granted, on the lower end with the “lesser” Ivies; interesting aside: in terms of alumni achievement, the low end of the Ivies is set not only by Cornell but also UPenn and Columbia, even though these days, kids seem to think that those 2 are almost HYPSM-status).
Not a big difference, but not exactly the same either.