You can't beat the Cummins power/upgradeability. The new Dodges are real nice, but the trans still takes back seat to the Allison, stock to stock. If your plan is to keep it stock or very mildly upgraded, and you have to have an auto, I'd go with the Dmax.
If you can deal with a manual, you can't beat the Cummins w/6-speed manual trans for towing.