I love an easy one right out of the gate on a Sunday morning!
Which one matters to you more? Not asking for you to tell us- looking for you to take a hard look at yourself. Bluntly: is getting into UPenn worth giving up an acceptance to MIT? ED only if you have one place that you will cheerfully jettison every other acceptance you could get- so only where you really want to go.
Given the kind of school you are in, and your level of achievement within that school, I can take a decent guess at how important the name of where you get admitted will be over the next 10 months. Here’s the thing: next May you will graduate, and the weight of that name will drop by hugely. Come August/September you will roll up to your new home and face the reality that you are going to live there, that those people will be your people for the next four years- whether that school was your 1st choice or your 5th choice or your last choice.
ps, how much time have you spent at MIT? It is a really strong ‘fit’ school- it suits you down to the ground, or it really, really doesn’t. Most of the students that I meet who are really excited about MIT have never been there and/or never met current students. If you haven’t had direct experience, get it.
pps, really Columbia and Brown? Have you read this page: The Core Curriculum
and clicked through to all the links?