Both great trucks. I am a pretty loyal GM guy, drive a 2013 Dmax, but always admired the Fords. Ford will have a nicer interior, and roomier. Can't go wrong here.

I have a 2011 crew cab chevy drw. Been in both. I will say the chevy is much more compfortable to tow with. Towing a 36 skater is a dream.My daily driver is a f-250 so I'm not really brand loyal either.. Power seems to be the same. The exhaust break on the duramax is in my opinion the tie breaker. It's really nice.

Hands down the duramax! Sorry l.v. But I've been a duramax guy since 05. I wish there was a truck with ford interior and the electronics ford has with the duramax drive train. I do not like the dpf thing but it doesn't stink at least. My 06 has better power only because I won't add the stuff I have on 06 on the 14 because of warranty. But here's the other side. The engine brake is kickass, also walk into any chevy dealer and ask to speak or see the Allison tranny guy. They usually won't have one because there's not a lot of Allison failures. But the truth is go with what you like but I lean toward the duramax,Allison any day.

Both trucks have ALL THE POWER you need.

Bonus is the Allison transmission.

I know i am a GM guy---- GUILTY

I haven't had a ford newer than 06, and I currently drive the exact GMC you are looking at. I have the black maxx programmer, dpf delete, cold air and 5" mbrp turbo back exhaust. I just turned over 98,000 miles (65K since 12/12 when I bought it used) I haul ALOT on the highway, and the ride difference between GM and Ford is Huge. I personally love the GM denali interior but it is a little cramped compared with ford or dodge( it feels more like a cockpit). When you are trying to change positions or spread out a little on a long road trip you are very limited. The fold up back seat with open floor is the best I've had for putting lots of big stuff in the back seat (or a dog) The leather is holding up very well.
I have done NOTHING but oil changes every 5K, fuel filters every 10K , tires once, and tranny/differential service once. I second the fact that the exhaust brake and Allison transmission is a deal sealer. I can usually put the tranny in manual mode and downshift to be in higher rpms, engage the exhaust brake, and never touch the brakes when towing lighter loads under 10K down 6% grades. Empty running 75-80mph I get a true 20+ mpg(23/24 @ 65), towing 5-10K at 70mph around 14-16mpg, and towing 12-13K at 70 is 12mpg. That is average and I have a lot of BIG hills here in the rocky mountains.
I love my truck and would buy the exact same one again if I needed to. I have owned 5 fords and one dodge dually, so I do have real world experience to compare them to.

Ultimately.............you should buy the truck that YOU love, because you will be the one driving it!

I had a 2011 6.7 and have a 2013 6.7 with no issues. The interior room is huge in the Ford compared to the Denali, and the Ford electronics are superior. Towing wise, not much difference. Ford seems to have more low end torque and GM seems to have more high end torque. Plenty of power in both. The deal sealer for me would be the extremely tight turning radius on the F450.

Yeah, my turning radius sucks. 4 lanes