Let me add to the recommendations for diesel. It doesn't cost as much as the retail price. Most savvy shoppers will only pay about dealer invoice, which makes the engine and transmission options cost less up front. Come trade-in or sale time, you'll get much of that back. In the meantime, you'll be saving fuel, and experiencing benefits that are priceless.
With a gas engine, the peak torque is at higher rpms, and it's less than the diesel's. The solution is to run one gear lower, which not only gets the rpms up at/near the torque peak, but gives torque multiplication, as well.
As John said, the diesel running at a lower rpm is MUCH less stressful on the driver than the gas engine screaming along. It's so much more relaxing. But that's not the critical advantage.
When you have to downshift below Drive (to 2nd on a 4 speed or 3rd on a 5 speed), to climb a long, steep grade, the torque converter is unlocked and slipping. This generates a lot of heat, often more than the transmission cooler can handle. A diesel seldom needs to do this, but a gas engine will, to get the rpms up near the torque peak, and get the same torque to the wheels.
With 3.73:1, my overdrive puts me at 2,000 rpm at 70 mph, and at the engine's 1600 rpm torque peak at 58 mph, the speed I usually tow at. Punching OD off at 60 mph brings rpms up to 2400 rpm, just below the 2600 rpm horsepower peak, which occurs at 65 mph in Drive. The newer diesels reach their torque and horsepower peaks at higher rpm, and are better suited for faster towing.
The increased mileage means more than cost savings. Those who opt for a Crew Cab often settle for a short bed to keep length reasonable, about the same as an Extended Cab with long bed. This means a much smaller fuel tank, and fuel tank size times mileage translates into range. The mileage of a diesel is, IMHO, really necessary for towing with a short bed truck, and it's just plain great on a long bed.
Another advantage, at least with the late-2001 and later Fords, is that the fuel filler is large and straight, meaning you can take the big high-speed nozzles at the truck islands. They're so much more roomy than even the RV islands at Flying Js, and you don't have to worry about nearby gas pumps generating fumes that would require you to shut the refrigerator or furnace off.
If you're gonna buy GM, get the Duramax/Allison options.