I think there are some important variables to consider here -- first and foremost, where do you really see yourself working? Do you want to be in the DC area? Then, I would probably say go to GMU. Yes, it's still the third fiddle in DC, but I think your job prospects are better in DC with a GMU degree than a UNC degree. There are a ton of GMU alumni in the DC area, not only in firms but also on Capitol Hill, government, etc. If your dream in life is to work in that kind of environment, GMU seems like the better choice.
If however you want to work pretty much anywhere in the south, then I would say UNC is far better -- without a doubt. If your not sure where you want to work or if you want to work somewhere outside of the south/dc, then I think the choice is much more difficult, though I'd probably give a small edge to UNC, simply because Mason is relatively new to the more elite law school levels. I do think though that both of these schools are more regional in nature though and if you want to work outside of DC/South, you should probably look at compartively ranked schools in those regions.
Also, I know you only mentioned the employment factor as the sole consideration, but it bears mentioning that the cost between these schools is huge, depending on whether you are in-state or out-of-state. Both schools have very inexpensive in-state tuition and relatively expensive out-of-state rates. You can't obviously live in both states so where you would get in-state tuition might be a very important factor....just my two cents.