New tow vehicle...6.4 hemi or Diesel?

We have both at work. I bought the 2014 3500 with the 6.4 because the def was a new thing and didn't want to run into issues like the 08 Ford we have. We tow several large generators ranging from 10k-13k. The 6.4 is a little slow off the line with the 13k but has towed it fine. Just this year I bought new 5500 with the Cummins. It tows the 13K trailer like it is not there. Both trucks were ordered with maximum towing in mind. The fuel mileage is negligible between the two but I believe that is due to the difference in the added weight of custom bed and gearing between the two.