The 6.0 will be mated to a 4 speed transmission. What transmission is behind the 8.1? I believe it was an option to get the Allison? If it had that, I would choose the 8.1 over the 6.0. That being said I have owned an 04, 06, 11 and now a 17 with the 6.0 with both 3:73 and 4:10 rear end ratio's. the 2011 and 2017 have the 6 speed transmission. the older trucks had the 4 speed.
I pull a 26' TT that maxes out at 7000 lbs. I have a leer high rise fiberglass cap on all the trucks.
The 6.0 is no slouch, it handled the 7,000 lbs and loaded bed fine in the north east united stated. That era engine, transmission will have to rev more to get in the power band. Not a problem as the 6.0 handle it fine. I would prefer the 4:10 ratio for the mountains.
If both trucks were equal shape I may lean toward the 8.1 for the shear HP and especially if it had the allison transmission. Mileage is not going to be to much worse than the 6.0.
I consistently get between 8 and 10 depending on where I am going when towing and the speed limit. Under 65 saves fuel.
