I am not sure if you made your decision yet. I have a 2012 GMC 2500HD with the Cast Iron 6.0L, 4x4 Crew Cab with 6.5 foot box. I have the 3.73 rear end. I wrote all of the above because you need to be really careful with the GCVW, my truck as configured is 15,500 combined max trailer + truck. My truck at normal operating without family and camping gear weighs in just shy of 6900 lbs. this leaves only 8600 lbs for my trailer and all the gear. We normally pack, bikes, firewood dogs and 4 people. Last year we did a 2400 mile BC to Oregon round trip. I hit an unmanned commercial scale just to check and we were within 600 lbs of our GCVW while towing our 1976 Overlander. It is a shame, not sure really how the GVVW is calculated as my trailer axles totaled 5,500 lbs which seemed very reasonable. Just be careful is my point, my father has the same 6 speed, same 3.73's in a 2008 version which has less power and his GCVW is 700 lbs more than mine. I can't explain it. While travelling Southbound on I-5 we hit some short but decent climbs around Grant's Pass area, the truck and trailer will lose about 10-15mph, I used to have a 2004 6.0; with the 4 speed and it didn't really like to rev passed about 3500 rpm. This new version, will pull all day at 4500 RPM without moving the thermo 1 deg passed the 100 mark. Our Oregon trip was in the summer 2015 where the heat inland was in the 100-110 deg range. Be prepared as others have stated 10-10.5 MPG towing, and 13.5 MPG empty everyday driving. I took my last one to 210,000 miles before I sold it, I still see it driving around 3 years later. No major repairs, the old belt and I think a water pump.
They are good trucks but on steep grades you aren't going to hang with the diesel guys. I would recommend the 4.10 gears if you tow a lot, it also negates my entire ramble above on the GCVW issue.