Well I go to Duke and my boyfriend goes to UNC (he actually chose UNC over Duke), so I'll chime in. They are very different schools. You first have to ask yourself what you want out of a curriculum...I love the idea of getting all our basic sciences over in one year and getting 3rd year to do research/dual degree/go abroad, which would (hopefully) help in getting a residency. Another huge advantage is that Duke not only puts you on the wards sooner, they give 3 elective periods and an elective within surgery. A lot of places don't even give elective time during their 3rd year, so this is tremendously helpful in gaining exposure to specialties early on so you'll have an easier time deciding before 4th year. On the other hand, I have to study A LOT more than my boyfriend does and I've definitely been plenty jealous of their curriculum when things get particularly stressful. So that's the first question you have to ask you yourself.
Second is the study body. Duke is smaller (102 vs. 160) and the student body tends to be a bit more intense. I love my classmates and we all do tons of stuff together, but they're a little on the intense side, which has been difficult for me to adjust to. My boyfriend's friends seem much more chill, which I tend to get along with better as far as "in the classroom" goes.
And lastly is price. Duke is $35,000 a year in tuition, UNC is $11,000 per year (although cost of living is higher in CH, but not $25,000 worth). Big difference. However, price is the last thing you should take into consideration, in my opinion. I have a classmate who turned down a full scholarship at UNC to go to Duke, whereas my boyfriend and others went the UNC route. If you like everything about the schools equally, then by all means go to UNC. But that's almost impossible to do given how different the schools are.
They're both great schools and you won't be prevented from going anywhere by going to either one of them. However, I should add that there is absolutely nothing like being in Cameron for a Duke basketball game. UNC SUCKS!