Max Tow rating is only half the story and is very misleading when it comes to pulling a fiver.
Total truck PAYLOAD is the number you need to pay most attention to.
Just because the truck can PULL 15000# or more does not mean it can handle a large fifth-wheel.
Not sure there are any 40'+ fivers that a LOADED 2500 series truck will be able to CARRY when the fiver is loaded up. The dry pin weight is meaningless, you need to figure out what it will be when loaded.
Truck in my sig has 2700# payload and uses 2500# of that (and is within 200# of GVWR) when pulling my 27' 9600# fiver.
Get the 3500! But be prepared that a SRW truck will still have limitations. Most 40' fivers really are in DRW territory.