I rode a vintage bike--an '83 Trek 720--when I did my cross country and it was more than fine for the ride.
That said I'd take the Surly for 2 reasons. Everything else being equal, I'd take the bike that can take a larger volume tire. You're not going that fast when touring (as compared to regular road riding) and larger tires make the ride more comfortable. Plus it will be easier to ride on gravel or bad surfaces.
Tire availability is not what it once was. I had a hard time finding a 700 x 32c replacment tire in North Dakota and more or less nursed the bike with a sidewall cut across the state until I found a large enough shop in Fargo. By the way, I had carried a spare tire but I got a bad cut in MT and had to use the spare. I was unable to find a replacement in MT and then got a decent cut in ND but I booted the tire and rode it that way until I hit Fargo.
To my mind, the choice boils down to tires. Everything else can be made to work just fine for your trip.
Last edited by bikemig; 02-09-21 at 02:08 PM.