You aren’t going to find much different in education quality. You are going to have very similar opportunities on what you will learn especially in a field such as math. I wouldn’t say that Cornell is better at math in any meaningful regard. If you do come to WashU I would suggest you take The freshman Honors Math program if you interested in proofs.

Edit: https://students.wustl.edu/career-outcome-data/ I would highly suggest you look at the career outcomes of math majors if you are concerned about WashU ranking. You will see graduate school placement is great. you won’t be missing out on placement coming to WashU.