I see no reason to buy a F250/2500 instead of the 350/3500 models for towing a fifth wheel. For the Fords, the only real difference is the rear springs and a few hundred dollars. What you get is about another 1000 lbs of payload. I am up sizing from an F150 that has been pulling my 9500 lb fiver comfortably for 3 years, but plan to do a lot more of it and want to be positioned for a heavier fiver in the future. The F250 would have only been an incremental up size.
Here in Vermont, the F250s tend to be used as plow trucks because of the softer rear springs.