I-81 in Virginia is not good. Heavy Truck traffic, worn out driving surface and during the summer, a lot of construction just to keep up with the road problems.
If you do get to Harrisbug, US-15 is really not too bad. There are stretches of old road to be sure but there are also a lot of Interstate like by-passes that take you around a lot of the old towns. US-15 has had signs for many years saying that it is the future route of I-99. It's shovel ready but somehow keeps getting overlooked.
If it was my choice, I would take US-15 over I-79 & I-90. The times are comparable, no tolls and it's a more scenic drive.