I never had the chance to compare the previous generation F350 SRW to my previous F250 but I can tell you for sure that there is a huge difference stepping up to my new truck, it "carries" the weight of my fiver a lot better and does not feel nearly as soft on rutted and rough roads. I think that in the Ford's case the extra Leaf makes a huge difference.
In my case the F250 would have been more than sufficient as I have a light enough fiver but when ordering the truck, its almost stupid not to opt for the 350 as its no more money to register in my area and its about the same purchase price.
My father was going to order an F250 as he only tows a light 30ft TT and I convinced him to move up to a F350 SRW just in case he ever decides on a fiver, well he's already talking fiver now.... good thing he bought the 350 can gave himself a lot of options.