I have a 2012 3500hd S&W with 4:10. My fiver weighs 12280 scaled. I will say that I am completely satisfied with the 6.0 gasser for my needs. I made a conscious decision to not buy a diesel because I don't tow full time and the extra $8k for the dmax did not make sense for me. Sure a diesel will tow effortlessly and would make sense if I towed full-time, but I am a weekend warrior. I have no problem with my set up in the hills of upstate NY.
Everyone will have their opinions about why their brand is better. If you want a GM and question the 6.0 don't think twice. It is a solid and proven engine.
The 3500hd SRW has a 10800 gvwr which gives me around 4000 payload.