GMC is a higher priced GM truck. Other than a few luxury items like interior or plastic, they are the same truck. I have a 2013 3500 4wd, single rear wheel duramax and love it; however, I have had several issues with the NOX sensor and radiator and going through the GM repurchase. But, I am staying with GM because the truck has never left me stranded, only warning lights have come on and GM agreed to give me a new truck. I have two friends who have about the same year trucks and neither has had any issues at all and love the truck. That is why I am staying with Chevy.
I would suggest you get the most current year as possible because in 2010 GM had to put emissions equipment in the diesels. Each year after they have made whatever part/technology improvements needed.
Towing is awesome, I love the exhaust brake system. I get about 18mpg non-towing and about 11-12mpg towing my 35' 5th wheel trailer. I will be going from a shortbed to a long bed for additional comfort towing, so you might want to consider all options.
Good luck