Where do you plan to travel? If you plan to run around the flat midwest, a gasser is fine. Major hills is another matter. The gasser will do fine for most towing, but they give top power at 3000 or 4000 rpm. The diesel gives equal or more power at 2000 rpm. As said, the diesel extra cost will come back in full, or close, when you sell it.
As for which truck, if you pick the FW first, start calculations with the FW GVWR, NOT dry wt, for the reasons stated above. Take 20% of the GVWR as estimated pin wt. Add 200# for a FW hitch. Add the weight of ALL passengers, pets firewood, toys and cargo carried in the truck. This is how much weight you expect the truck to carry. When you go truck shopping, make sure the payload for THAT TRUCK, according to the yellow placard on the driver door latch pillar on THAT TRUCK is greater than what you expect to carry.
As a general rule, 2500 models have a 2200# payload. 3500 SRW, about 3500# and 3500 DRW about 5,000#. More options or higher trim level, less payload. Fewer options or lower trim, higher payload. The advertised payload is a base model, buck naked truck with max tow option only. The yellow placard applies to the truck to which it is attached. They have been required since 2009.
Do your homework and be safe and properly equipped. Never believe the salesman.
