With the 6 vs 10 speed argument a trend I see especially with GM is the more transmission gears the higher the rear gear ratio. I have had several gas 1500 trucks. A 5.3L 4 speed with 3.73 rear end, pulled really well, not the best mileage but acceptable. A 5.3L 6 speed with 3.42 rear end. Pulled acceptable and mileage was decent. 6.2L 8 speed with 3.23 rear end. Pulled somewhat lack luster, but 6.2 made it tolerable and milage was great. I understand my 2500 is a different platform made for pulling /towing. I find the 6.6 gas with 6 speed and 3.73 to be the best combination for pulling/towing for me with acceptable mileage.
The diesel vs gas debate gets plenty of fan far, but for me the up front cost difference, maintenance cost and the cost at the pump means I can drive a lot of miles before, if ever making up the cost differences.