Thanks for the reply DrewE. So it looks like both of the routes use I-78 to get onto I-81. The "shorter" route says to stay on I-81 for almost 500 miles, passing through Maryland, WV, VA and then entering TN.
The other route says I-81 for 68 miles to enter Maryland, then it's I-70 for 25 miles, I-68 for 111 miles to enter W. VA, then I-79 90 miles, US-19 for 67 miles, then I-77 for 67 miles to enter VA and back to I-81 148 miles to enter TN.
We'll only be on I-95 early in the trip at the GW bridge which if we don't time it right I'm sure will be major traffic!