The F250/2500 are artificially capped by classification at 10,000 lbs GVWR which make it look like they have low payload. They typically are just as capable as the F350/3500 SRW if you configure the options correctly. If you live in an area where heavier vehicles are not allowed to park or there is significant higher registration or inspection cost, it is an option.
If you do not have these restrictions, start at the F350/3500 SRW and configure it for the maximum payload by selecting the highest rated tires/rims and heaviest suspension.
If you decide on a short bed, you may need a slider hitch to safely back up a fifth wheel and not cause cab damage. This hitch will be more costly and heavier than the basic fixed model, so try to get the long bed. Some manufacturers use a smaller fuel tank with the short bed (this was my biggest peeve of owning one).
Get the highest ratio axle gears available for a gas engine since it makes power at higher RPM's. You will get slightly worse unloaded mileage, but a better towing experience. If you later find that the gearing is too low, you can always upgrade to a taller tire when the OEM's wear out.