Tow rating rarely limits the size of 5th wheel you can pull. Payload does.
The engine and transmission is essentially the same, so how much pull isn't much different (dually may have a deeper rear diff ratio to give a small boost).
Payload in a dually is significantly heavier, so less likely to hit the max payload before hitting the max tow rating.
Long vs Short bed...long bed is definitely preferable. Short bed, you either need an expensive specialty hitch or you constantly have to be careful to avoid hitting the truck cab.
So what is the trailer you are planning to pull (with GVWR numbers)? If the SRW is capable, I would take that over a dually.