New tow vehicle...6.4 hemi or Diesel?

If it was me, the gasser, especially if you aren't towing that trailer other than just to race X number of times a year.
The gasser is cheaper to fix if it breaks, and for the money you save by not buying the diesel, it would take years to pay for the difference in fuel.
A 2020 6.4 mega cab 4 by 4 even with 3.73 gears is rated right at 14,000 pounds towing capacity