Id look into an 06/07 Mega. Its easy to get big power out of the Cummins, just as easy as a Dmax. Sure, GM as EFI, so initially you can get more power than a Cummins. But to get any real reliable power, your gonna have to get a modded single or dual pumps, regardless of engine. And a modded trans in either. Modded, Id take a 48re over an Allison anyday. For racing and all out power, its a better trans suited for that type of use than the Allison is. The Allison is basically a towing trans.
On just tuning, with a modded pump and trans, the Duramax can go higher than the Cummins. To get to the power that EFI is capable of, the Cummins needs injectors and turbo(s). BUT, there is a limit to the Duramax. The internals arent close to being as strong as the Cummins, so your gonna have to pull it and build the whole thing. General rule of thumb is much over 650, your pushin your luck...
In the end, your gonna sink the same cash into both in order to get to the big show...maybe even less for a Cummins.
As for the trucks, hands down the Dodge is far better a truck than the GM is. I used to be in the body repair business, and the GM trucks are just about the flimsiest things on the road. Ive seen slight fender benders result in replacing the bumper, grill, radiator support, intercooler, radiator, fenders, all the bumper brackets, frame straightening, etc. Same situation in a Dodge is bumper and grill, MAYBE a bumper bracket. Body shop had GM bumper brackets IN STOCK, cause theyre such junk. You never see a body shop stock parts for vehicles...dont get me started on how the whole truck is assembled :vomit007:
For the ride, honestly, I bought a truck, I want it to ride like one. I really wasnt impressed with the GM trucks ride, especially with a lift. Lift it, and your rides out the window.