As a 3rd vehicle I'd go used for a low optioned tundra with the big motor or a similarly optioned 4th gen Ram 2500 with the hemi or as new as possible 2500 chevy.  Ford might be a good option, but I don't know their gas line well at all.

I have known many guys with the new hemi and if they keep up with oil changes it is a great motor.

I would avoid diesel unless you find a great deal.  The cost savings just isn't there for the weight and frequency of towing.

No idea on the new chevy motors, I would have to drive them and compare.  For power curves, I would want to look for something flat and torquey rather than lots of peak power and high RPM power.

For Dodge, I am a diesel guy, but the new 66RFE found in the gas 2500s and the 68RFEs in the diesels have been pretty good for the most part when not hot rodded.  The older 48REs are good too, if maintained and not hot rodded.  Most of the older dodge transmission woes came from the fact that they needed maintained like the 70s transmission they were, I have 500hp going through an ATS 47RE in my truck and it runs great... But, most people viewed these trucks as muscle cars and never changed fluids or adjusted them properly every 30-60k miles.  From friends and what I have read, the Dodge transmissions have had some issues that are typically fixed by a reflash of the computer, although there are so many sensors in the new trucks that some have had problems there.  I would do some reading and ask some mechanics/trans shops on this if I were considering one, but I wouldn't shy away from a good deal if you do your home work on the transmissions.

On a towing note, I love power when towing.  However, I don't think that power is the cats meow when it comes to towing.  For me, stability is what I want.  Towing, especially larger loads close to the max rated GCVW of the vehicle, should be done slowly.  65 is kind of my max speed when towing heavy (20,000lb car hauler) and I want the whole load to be completely controllable by me.  I want to be able to turn, stop, and quickly control any fishtailing caused by heavy winds or semis.  

If you do decide to move up to a heavier 3/4 or 1 ton truck, you will have increased costs with the increased stability.  The stability comes from the heavier frame, the heavier suspension, much bigger brakes, and stouter driveline. But, these heavier brakes, tires, and all will cost more to maintain.  Diesels are expensive for oil changes just due to the large volume of fluid changed.  I would avoid diesel, especially as you don't have a lot of mountains in your area and aren't towing anything crazy.  Plus diesel parts are expensive for anything Mopar built after 2002.