Can't speak to DEF luckily my Duramax is pre DEF.
I have pulled my 38ft 5th wheel with 4 different trucks... two were duallys and two were SRW. The duallys always felt more stable.
Both of the SRWs felt as if the front of the 5th wheel was pushing the rear of the truck when cornering, kind of a squishy feel. Tail wag the dog... The duallys never felt that way.
While towing the 5th wheel, I've had one rear tire blow on the dually after hitting a rock while crossing a bridge in St Louis. No effect to the stability, I was able to slow down and get across the bridge and pull off to change the tire. At 55MPH I wonder how the SRW would have reacted with a blown rear tire on that bridge...can you say Davy Jone's Locker. Just kidding it probably would have been ok as long as I didn't panic and slam on the brakes.
It never even crossed my mind to buy a SRW when shopping for my latest truck. I like the stability of the dually.
By the way all four trucks had long beds. We carry too much stuff to go with a shorter bed.