THe real issue is the width of the two. But from my own experience changing out a SW truck from a 6400 lb spring to an 8400, then buying a dually, I noticed NO difference when towing my 12K equipment trailer with 1500 lbs of HW with the 8400 lb rear spring SW setup, vs my dually with 8400 lb springs.
BIG trucks, ie semi;s, are going to singles more and more, get better fuel mileage, braking, cornering etc. BUT, they are also the same width! THAT is the key, not the 2 vs 4 tires etc. The ONLY reason for duals is the human part of the equation. If it were me, and I could get wide singles on the rear of a pickup, I would take that WAY the heck before a dually setup, assuming I could get both to be the same width. Even a sw setup with the same GRAWR of a dw setup, Would be the preferred over the dually, for ALL of the same reasons I described above.
Marty