Ric:
If you search the old posts, there are a lot of discussions of pickup truck preferences. I have 130,000 or so miles on a 1989 Chev 1500K with the old 350 small block with a TH350. A lot of that has been towing horse trailers. I have also spent a fair amount of time in a 454 Suburban, towing a 25 ft. race car trailer. Given appropriate gearing, a gas engine tows extremely well. It is quieter, smoother and doesn't smell as bad as a diesel. It does require more fuel, but you have to drive a lot of miles to make up the difference. Modern diesels develop more power for their weight than older ones, so differences in repair statistics don't favor diesels as much as they used to. Repairs on a diesel are generally more expensive.
Although I prefer gas, I think you will find that the devotees of diesel are passionate in their preference. I suspect that you won't go far wrong with either, but when you are looking at a particular setup, post again. Someone on the board will probably have some experience with a similar truck.
Charlie Iliff