FWIW--I have a 2011 GMC with the Duramax/Allison, awesome, pulls whatever you want. I've only had one problem, the computer that controls the DEF needed to be reflashed, other than that, nothing. Approaching 30k for miles.
My hunting partner was a diehard GM guy, dealer here upset him, went to Dodge, think it was 2007.5, can't remember. Anyway, 2 transmissions later and numerous other issues he traded it in on the new 2011 Dodge, been to the shop countless times, believe the tranny was rebuilt, now he just drives it with "check engine" light on, LOL. Some people never learn. He is my "brother" though, and you have to love him. LOL, but I rib him every chance I get.
2015 GMC 3500HD SRW 6.6 CCSB (Daily Driver/TV)
2012 Jayco 25 RKS
Living the dream in AK!