I own both trucks..............'02 F550 crew cab dually pickup conversion with a 4.86 rear gear and an '06 F350 crew cab dually with the 4.30 rear gear and "tow boss" package. I know that the factory GCWR is pretty much the same for both trucks but I think that has more to do with CDL requirements. The Dana 110 rear, steeper rear gears, heavier frame, bigger brakes, and 19.5 wheels with the G load range 14 ply tires all make the 550 a more capable tow vehicle when you get into the bigger loads and still want a truck that's the size of a regular dually pickup (vs an F650). With that said the F350 with tow boss is a great tow vehicle and probably enough truck for 90% of the boats that the average guy will tow.
Here's a link to another thread.........post #7 has some details about my experience towing 20k lbs + with the F550 vs the F350
Trailering 400 SS