IMO - unless you want to stay in St. Louis specifically afterwards, I would go with UNC. I say St. Louis because you mentioned that STL>Chapel Hill, but I don't think that should override the fact that Olin's small class of 140 compared to UNC's 280 will mean a weaker network. 280 is still a small enough of a class where you'll have smaller classes and be able to get to know a lot of your classmates. $26K is really not a lot to take out in loans...that's like paying off a car.
I prefer smaller schools, but I think when you dip below 200, you're getting too small in terms of network and on-campus recruiting resources.
Best of luck with your decision and any other apps you prepare.