I've owned two Cummins and one Duramax. First truck was a Cummins, second was a Duramax and I got smart for third and went back to the Cummins with a six speed. I love the low end power of the Cummins. The Duramax turns a lot more RPM's in general. In my opinion, less revolutions = a longer life span.
Allison transmission - I may be wierd but I really didn't like the braking feature downhill towing. 4000 rpm's for an extended period downhill made me very uneasy. A diesel at those revs just doesn't sound natural.
Until Toyota comes out with a diesel in a one truck I will continue to buy Cummins.
I vote you g with a dodge and when the truck falls apart around he drivetrain I will take the drivetrain off your hands and put it in my 97 Burb LOL.
I have always been a GM guy but the aftermarket seems to be stronger for the dodge. It is a tossup either way.
Big question is do you want a 2WD or 4WD? If you want 4WD i would go with the solid axle dodge. If yo want a 2WD or a good ride go with the GM. As far as drivetrain I think the equal out in the end. The GM has a strong trans and the cummins can't be beat.
Crazy thougt; Buy a GM drive it for a few months until it gets totaled then buy it back. After buying it back part it out except the trans. Go buy a dodge with a bad trans and put the Alli behind the cummins LOL. Crazy thought ends now!