taken me a while to get to your question, Semper, but, I'll give it a go. I think there are 3 reasons why somebody might pick NC over SC.
First, there is a tendency to value "big-time experience" come the tournament. I think even the selection committee suffers from this, as they seem to prefer second-tier teams from strong conferences over first-tier teams from mid-majors. NC has spent a whole lot more time as a "national power" than SC, and the NC/Hatchell program has played in a lot more high-visibility/TV games than SC/Staley. SC still has a bit of an "untested" quality.
Second, even though both conferences are power conferences, I think there's a tad more respect on the BY for this year's ACC - ND, MD, Duke, NCState, NC, Syr, etc. - than this year's SEC - Tenn, KY, SC, A & M,LSU, GA, Vandy, FL, etc..
Third, it's really hard to deny the sheer athletic talent on the NC team. Yes, they're young, and, yes, they seem undisciplined at times, but, geez, when they do click, it's impressive. Plus, lots of BYer's coveted Diamond, so, we know more about her and NC than we do SC.