Hi! Congratulations on being accepted to both programs!
I myself am deciding between U-M and UNC, but for a completely different program. But I am from Durham, NC and went to UNC as an undergraduate and can assure you that it is an amazing school and an incredibly welcoming community and learning culture. If you find at any point that you don't like living in the small town of chapel hill during your PhD, you can always move to Durham or Cary and those are both really diverse areas nearby! Lots of people around the world move here for Research Triangle Park, Duke, UNC, NC State, etc. I have a lot of international friends. Plus, it is warmer here, so people actually hang out outside, hike, go to the beach, etc!
The only reason I'm on the fence is because I want to try something new after being in the "Triangle" for 27 years and I'm not involved in the sciences! But you can't go wrong -- you have great options.
#goheels