New tow vehicle...6.4 hemi or Diesel?

I tow with a 2015 5.7 Hemi regular cab 4x4 up to 11,400 lbs and it is adequate, but some of the mountains it is slower than I want. Be aware the 6.4 is recommended to tow with Premium while the 5.7 is 89 octane. They don't put the 5.7 in the 2500 or higher anymore, and all the new ones have the 8-speed, but mileage is not good. My truck gets 7-8 mpg towing the heavy weight, and 12-13 with an open trailer. My buddy has a 2021 regular cab 4x4 with the 6.4, and he gets about the same mileage towing 7-8K lbs. This is real world, so I think you would be happier with a Cummins. BTW, I have 278K miles on my truck with minimal issues except my A/C died while it has been unGodly hot out...