Hi John and welcome to the RVUSA Forum. The pros of running dual rear tires - increased load / tow capacity and increased traction. The cons - 2 extra tires to buy and check pressures on and increased rear width of the truck. If I was going to be towing a fifth wheel or even a large travel trailer, I would want the dually. JMO.
I have towed with both in the past. Give me a dully for anything over 30 ft. Downside of the dually for me was having to drive around when not towing. Hard to park in some towns. JMO