Get a 6-speed Cummins. Or 5-speed if you can't find one of the H/O 6-speeds.
Or if you must have an automatic, Ford will be fine.
The Dodge will get the best fuel economy and has the most power potential, as well as being the most durable. Cummins designed the engine for actual medium duty trucks (think straight trucks, chassis cabs, etc) and Dodge shoe-horned it into a pickup. The Powerstroke and Duramax were designed for light duty truck use, and adapted to medium duty trucks.
Ford even offered the Cummins as an upgraded engine choice in the F650+