New tow vehicle...6.4 hemi or Diesel?

there is nothing worse than listening to a gas truck screaming down hill trying to slow down from 10,000 feet to ground level, VS Running 1800-2400 under control at a leisurely pace
with the other diesel trucks out there is reassuring. I towed with gas trucks for years until I could afford a real Hauling vehicle.
I never got less than 11 MPG pulling 20,000 lbs, and averaged 14-16 pulling 12,000. No gas truck can come close.