Since about 1999, and up through 2007 as far as I know, the F250 and the single rear wheel F350 have had the same suspension. They both have had "auxiliary" rear springs, which in my case didn't even BEGIN to come into play until I had 2300 pounds of roofing in the bed. And then, only one end was barely touching its stop.
The difference between them is only on 4WD models, where the F250 has 2" blocks between the axle and springs and the F350 has 3.5" blocks and sits higher. Meaning you need more drop in the hitch. The single rear wheel F350 rides no rougher than the F250.
The dually is an entirely different story, and I wouldn't pull our Airstream with one unless I either had an air hitch or always had the 800 pound Harley in the bed, along with other stuff.
You can carry a motorcycle at an angle in a short bed, but I prefer to have the long bed. It also gives you a lot larger fuel tank for increased range.