It would probably depend on what part of South Asheville and how far from I-40 you are as to which route you want to take.
The last few miles of U.S. 321 coming into Gastonia
does have a couple of traffic lights, but nothing compared to U.S. 74 going through Shelby.
If you do take I-26, I would suggest taking it to I-85 near Spartanburg, S.C. instead of U.S. 74. The added distance is only around 15 miles, which is much better than the two million traffic lights in Shelby.
U.S. 74 is a great road with little traffic from the I-26 split to Cleveland County. It only lacks wider shoulders to be classified as an Interstate highway. If they would just hurry up with this...
R-2707 Project Breakdown Map