I'm reading that that sales person said the right thing,it all depends on the truck.
I'm a pre def diesel guy with a 05 Ram dually due to my 5er weights. When Ram beefed up the frame in 2010 a 3500 single wheel would handle my needs. If I find the right one I'll move up. A DPF doesn't bother me but no way I'm doing DEF. Cummins started DEF in 13 IIRC. Also you get a real crew cab starting in 2010, although the quad cab is sufficient room wise.
My 05 dually has a rear axle rating of 9350 lbs when a 3500 single wheel in the same configuration is 6010 IIRC. Starting in 2010 the SRW goes up to about 6500 lbs and that gets me to a single wheel. I sure would like to lose those hips, the fuel penalty of 6 wheels, and the price of tires at 50% more per set.
I've been on more than one CAT scale and did the Escapees smart weigh. I know my weights.