@deadgirl First, you are really smart and really good. Both Columbia and CMU would be lucky to have you. You would do great in either, and both have wonderful opportunities and great advantages.
I am, however, telling you that, whichever you choose, you will regret it, because of buyer’s remorse, but only if you start obsessing over your choice.
What you should do is, when you make your choice, follow this rule: absolutely no looking at the other college’s website, no looking it up on the web, no looking at their emails or brochures. Nothing. It no longer exists.
Instead, start going through the website of your chosen college. Look at all the cool stuff they have, all the cool courses. Read the stories of the students who go there. Obsess about it, indulge in daydreaming about how great it will be when you are there. Announce it on Facebook, and let the congratulations roll over you. Buy their sweatshirt online and have it delivered ASAP.
Because, you know what? Whatever choice you make, you will be attending and amazing college.