Once again another thread full of everyone's brand loyalties and biases.
OP- good luck with your choice. I have owned GMs my whole life. They are not the unreliable non-capable junk that the posters in this thread would have you believe.
The Duramax is on the same plateau as the Cummins. Many on here would have you believe that the Cummins is somehow superior, and that is just not the case.
I'm going to bet that if you choose the Duramax, or the Cummins that the engine will be the last thing you'll have to worry about. Almost without fail anyone on this forum trashing the DMax will be a Cummins owner. I don't know what motivates these guys because the evidence doesn't support any claims.
Where you should be focusing your attention is on the rest of the truck. Things like the HVAC, the front suspension/steering, how will you feel after driving it for 500mi continuous........
In short, the mechanical aspects of the engines are a wash.