Hey! I figured I'd chime in. Sorry, I'm so late to post.
kailun wrote: At UNC, I have a professor who I am interested in working with him, but I am not sure if my research interests will change once I take grad level courses..
This I think is a VERY important part of your post. You probably don't want to pick a school just because one professor is really interested in you. What happens if they change schools? Will they take you with them? What if you don't mesh personality-wise when you're doing research? How good are the jobs that said professor gets for his or her students?* What happens if you change research interests? These are all big questions. There's a lot of risk involved in these aspects specifically if you're committed to a single professor from the go.
If you're uncertain of what you want then it's probably advisable to go to the best school you've got an offer provided it has faculty in all the areas you're interested in AND their faculty on the whole tends to get their students good jobs. I have friends at a few name-brand schools with sink or swim atmospheres. Sure, if you're there and you're a star then life is great. If you're not a star, however, then you've probably wasted five years of your life in a really shitty environment just to get a piece of paper with a university name on it that'll impress people at cocktail parties. That being said I'd suspect Brown has a pretty collegial friendly environment.
Regardless, you've got two kick ass programs to chose from so I'm sure it'll work out either way
*For example, Dilip Madan is a huge name in math fin, but he appears to be pretty mediocre at getting his students good academic jobs.