jrob,
To answer your question, If you're going to do serious towing, buy the Dodge/Cummins.
First off, you are buying a real truck engine. Inline-6 has 7 journals for 6 cylinders, V-8 has 5 journals for 8 cylinders, and loads are much more balanced on the journals. Much better for longevity (most semi's are I-6's). Hopefully someone can post the pictures that show the difference in component size between the Cummins and the others. There is no comparison nor doubt which engine is built for trucks. Also the short stroke V8's are meant to wind up higher to get their power. The D-Max may feel faster dry, but for pulling a load you want the engine that will give power down low.
Don't be sold by GM's hype about having the strongest engine. The D-Max is really tweaked to get what power it has, and can't go much higher without melting down. The Cummins is really choked-down to get the emmisions ratings Chrysler needed. For very little money (especially when you look at the price difference between GM & Dodge), the Cummins is easily capable of much higher numbers than Dodge gives it (or GM could ever dream about).
The aluminum heads in the D-Max are still very much an unknown. They have had a lot of problems with them so far. Even if they claim to have worked this out, there is no history to prove it. The Cummins is proven to be good for hundreds of thousands of miles.
Look in the engine bay of both, thinking of how easy it would be to work on them. You will save whether you do your own work, or pay to have someone else do it. With the help of this website, you'd be suprised how much you can learn/do by yourself.
You should be able to find a ton more information by using the search function above. After enough reading, you will have little doubt which way to turn. And, when you're ready to order, you should have little problems picking options.
Good luck!