Duramax vs Cummins, well I have had them both. So comparing apple to apples I will compare my 06 LBZ Duramax to my 07 Cummins 5.9.
Injectors failed, 6 on the Cummins 0 on the Duramax.
Transmission issues, too many on the Cummins 0 on the Duramax
Times stranded, 4 on the Cummins (multiple nights in a hotel) 0 on the Duramax
Overall reliability, Duramax/Allison hands down
Fuel economy, close but slightly better on the Duramax
My Dmax had 285,000 on it when I traded it. I was lured over by the MEGA cab. I ran the Cummins for 394,000 it just left a month ago.
Now some things you need to know about the Duramax. The original to 04 was an LB7 engine and was an injector eating monster. Next came the LLY 04.5 to early 06, good engine but did have a small number of people that had overheating isssues. 06-mid 07 was the LBZ, best engine in my opinion, after this you venture into more emission controls. 06 and up also get the 6 speed Allison.
Now back to the Cummins, if you stay pre common rail (02 and older) now were talking. I've had 6. 2 24 valves and 4 12 valves. Downfall of ALL of these (except the NV5600 6 speeds) are the transmissions. Big weak link. One other weak link is the lift pump, if it fails it can kill the injection pump, an aftermarket lift pump like the Airdog solves that.
It all comes down to money 15k and up a Duramax LBZ, you'll spend well over 20k for lower miles. Under 15k and you can get a nice 2nd gen Dodge, and if your not afraid of miles 10k can buy a decent 2000 or so.
My next toy towing rig will likely be a NV5600 equipped 2000-2002 Dodge in the 10k range. Just can't justify the 20k for a toy towing rig.