Owning a 2001 8.1 340 HP/455 lb-ft, don't believe all the bad you hear about fuel use. From empty to 17K it still gets 12 MPG, on the highway, at legal + speeds. But that is with 4.10 gears instead of the normal 3.73s. That change alone adds 2K lbs to the towing ability. As for comparing it to the 6.0, forget it. Apples to oranges as far as torque and the 6.0 doesn't have the Allison. Plus the 8.1 is a 250K BIF rated engine. Some produce up to 95 lb-ft more torque than some of the 6.0 engines, or about 1/4 more torque. In the hills, you'd notice it. The 8.1 will run 3200 RPM up a 5% grade at 65 MPH in 4th with a GCVW of 17K. The 6.0 would be more like 4500 RPM to get the power and at its max load maybe down in 3rd gear. And getting no better mileage.
PS Topsite, the GM Vortec 8100 is 496 cubic inches.