From the 2020 Ford-heavy-duty brochure, there is a ovelap in payloads for the F250 and F350. (These figures may be slightly different for 2021.)
There are many GVWR choices: 9,900 up to 10,800 for the F250; 10,000 up to 12,400 for the SRW F350. Adding 4-wheel drive to either reduces the payload by 400 to 500 lbs. And, for some reason, adding a high-capacity towing package to the F250 reduces its payload.
With so many variables (GVWR, regular or crew cab, short or long bed) there is considerable overlap between the highest F250 and the lowest SRW F350 payloads.
For the crew-cab short-bed preferred by many RVers, the numbers for the F250 with tow package are 3,750 (4x2) or 3,320 lbs. (4x4).
For a comparable SWR F350, the numbers vary from 3,650 (4x2) or 3,220 (4x4) up to 4,460 depending on the choice of GVWR.
For a crew-cab long bed, the numbers for the F250 with tow package are 3,520 (4x2) or 3,040 (4x4).
For a comparable SWR F350, the numbers vary from 2,990 (4x4 with lowest GVWR) up to 4,730.
Bottom line: the SWR F350 payload for a given configuration is generally (but not always) about 500 lbs. greater than that of the F250. The payload depends highly on the configuration: GVWR, choice of engine, cab, and bed.
For maximum payload, the F250 short-bed, regular cab and no tow-package can have payload up to 4,260 lbs. (4x2) or 3,820 (4x4).
The SRW F350 short-bed, regular cab can have payload from 3,630 up to 4,960 lbs, depending on the choice of GVWR.
To make a choice between F250 or SRW F350, it seems that one should first decide about the cab, the bed, 4x2 or 4x4, and the engine. Next, we need the required payload: hitch weight, weight of fuel, other things in the bed and back seat. Add a few hundred pounds for good measure. Only then can you make a choice.
ps: my 2008 SRW F350 is showing signs of age (not with engine or transmission, but with various other things), so I am trading for a 2021 SWR F350: a special order to get exactly what I want, and hoping for delivery in February. That's why I have been looking so carefully at these numbers.