I went from an '06 GM dually C/C long box Dmax, to an '11 3500HD Chevy SRW C/C Dmax standard box (6-6) and it tows my 15000lb fiver better.
I can only say from a GM owners point of view, but in '11 GM made big changes in the frame, brakes and suspension. (same body style)
The GVWR for the 3500 SRW went from 9900lbs, to 11500lbs, which is 100lbs more than my '06 dually.
The changes, including factory exhaust brake, trailer brake controller and trailer sway control made this truck more stable all around than the dually.
And yes, I'm within all weight specs with a 2850lb pin weight and loaded truck.
The question of which truck (2500-3500-3500 dually) is better really blurs with new technology being introduced.