I have done both, tho not to Lorton (I was going Atlanta to Boston). I quite enjoyed the I-81 route, though there were a lot of trucks on the Virginia stretch, fewer once I got into PA, which is where you would be.
On the weekend, I agree, the traffic would not be bad, but for me, I'd opt for the scenic route, even if it were 30 mins longer since just one traffic jam on 95 will easily eat that up anyway.
Nevertheless, my family will be doing the drive in February on a Thursday night, then back on a Sunday. In the middle of winter, I think we'll do the coastal route (tho in on 84 to the Mass Pike), unless there is a coastal storm brewing; in the snow, I'd rather not be in the mountains, unless I am avoiding a storm.