My choice would be a cummins or a newer duramax.
The powerstroke motor is absolute international garbage. they seriously suck ass. its not a matter of if they blow but when.
Cummins has transmission problems. In the infinite cost-cutting wisdom of the Chrysler cooperation, they chose to mate the cummins with a gasoline transmission barely sufficient enough for the torque rating of a hemi. The tranny is the weakest link and probably if stressed often enough, will give you problems.
Personally, I'd rather buy a cummins and spend $2k to beef up the tranny than spend $6k to replace the Ford's lousy excuse for a diesel motor.
Or, buy a duramax with an allison. Its a good stout transmission. The motor is decent, much better than the ford, but its not a cummins.