Go with a gas engine and avoid diesel at all costs. It costs more to repair a diesel engine than to completely replace a gas engine in a truck. It is also more difficult to find stations that pump diesel on the highway if you venture off the interstates.
In terms of reliability I would go with a Toyota Tundra and know I had the most reliable truck I could buy. Choosing between Ford and GM I would check the Consumer Reports stats on repairs by type by model year.
A truck may have had a lot of problems with electrics or the cooling system or transmission and these are often expensive to diagnose and repair. For towing the reliability of the transmission and the cooling system are important.
I would also want a 4.10 rear end gearing and a 5 or 6 speed transmission. The extra speeds are overdrive gears which don't do much when towing but do improve fuel economy when you are not.
