Check out TFL.com for towing vids for all 3 gassers. Yes, I know these are up more extreme inclines and other things, but I still think they shed some insight into the overall capabilities of each.
Test drive all 3 for yourself, because the interiors are definitely different for each.
I love my Chevy 6.0. I really like the interior, love the looks, and it tows very well. Yes, the 6.0 is outdated, but it works great and its been proven to be EXTREMELY reliable.
In the end, get the one that suits your tastes and budget. Really can't go wrong with any of them.