As much as I would like to have an old 12V Cummins for the reliability and simple factor, I need a crew cab to fit my son and my dog. Not to mention I'd like to find something with relatively low miles and be nice to ride in.
As far as the trans issue goes, it seems everything out there has someone complaining that the transmissions are crap, with exception of the Alison. Keep in mind I currently tow with a toothpick Dana 44 and the same auto that can be found in the 350Z. Some guys blow the Nissan transmissions sky high on their first trip, while others tow quite a bit with no issues. I figure if you have a good trans cooler, have a trans temp gauge (and watch it), and drop down a gear to keep the TC locked (1.00:1 is what I am always in on flat surfaces) then your trans should not grenade.
Once you've lived with the torque of the diesel you will not go back to the gas platform. The Ford has the biggest most comfortable cab and best ride of all three. The Dodge rides hard the Chevy bounces. I have a Chevy, I have a Ford and I've borrowed my friend's Dodge. I will never give up my Ford and I will never buy another Chevy/GMC. All three are about the same as far as repairs maintenance but they are maintained very well. The Ford is by far the most stable towing platform I've used for my 35' 15K lb. horse trailer. No real fuel mileage advantage to any of them. The Ford's axles, braking and suspension is bigger heavier and stronger. My 99 F350 has the best braking and stopping power by far also. My Chevy and my buddy's 06 Dodge will not come close to stopping my trailer as short as my Ford.
Smart driving, cool trans temps, good fuel and all three will perform fine. What do you want in comfort, and stability?
A lot of people don't like the Ford 6.0's but with the correct aftermarket EGR cooler and an engine stud kit they are fine. Just stay 05, 06 or 07. There are plenty out there in spectacular shape and done already.
The newer trucks get poor mileage compared to the 07 and earlier trucks.