My uncle has a 2012 Duramax, 4-door, shortbox, auto. My dad has a 1991.5 Turbo Intercooled Cummins, single cab, longbox, Getrag 5-speed. Both are bone stock (well, I turned up the fuel a little on the Cummins, but not even enough to smoke). With a 5-foot load of wood in the Duramax and an 8-load of wood in the Cummins, the Cummins out-pulls the Duramax every time. Add to that the fact that the Cummins has about 10 miles (out of 100k) without a load and gets roughly 27 MPG in town, while the Duramax gets about 20 (it has 30k miles on it).
His pavement princess Duramax can't touch the Cummins. In my humble opinion, if you're going to get a diesel, get an old diesel. 1st and 2nd gen Cummins or an early Powerstroke. Chevy went for mass appeal, not power or durability.