WVa much better chance of snow than Brevard in March. IMO NC is better mtb, but mostly because the trails are more technical in WVa and I like some fast smooth dirt thrown in and WVa is lots of rock gardens.
Bring some fat tires which ever you choose.

Looks like average temp in March was 58 for the high and 36 for the low. So the mtn tops might be 10* cooler on top. Definately do able but throw some wind and rain in there and it could be uncomfortable. Parts of the Blue Ridge Pkwy are closed until March I think. So there maybe some snow up there.

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TIME RXR

No, it won't. Our team sponsor is a training facility out there and we've done camps in March there the past 3 years. Never had snow. It can be cold, but nothing accumulates. Last year there was one day with weird flurries in one of the valleys, but wasn't bad (mid Feb). Anywhere around the Lost River Valley or Cumberland Gap would be fantastic. On the other hand, if you want to fly into a closer airport anywhere in the Blue Ride mountains area would also be pretty solid.