We went from a half ton GMC to an F350 with a diesel. We had a Creekside 23BHS and the difference was amazing. We literally towed that trailer all over the US over the last two years. Mileage went from about 9 with gas to 13 plus with the diesel. Wind, hills, etc made no difference with the diesel. We have since gotten a bigger trailer and that would never have been possible with the half ton.
As to 4x4, that is a matter of weather, type of towing, and where you live. We live in Idaho and have snow and ice. Resale here almost requires a 4x4. However, outside of a March trip through Wyoming, I have never needed 4 wheel drive towing our TT. We also do not do a lot of unimproved road towing. We do some rough road exploring after we unhook the TT at the park. For that, the 4x4 has been nice. Just my .02 cents.