I priced out a platinum f450 and a platinum f350 drw diesel and the difference is $2500. The tires are very similar in height on the two trucks .... the f450 is about one half inch taller and three quarters of an inch narrower. Before this thread I actually thought the f450 was supposed to be in a class of its own but in the end I came to realize that there certainly are people in the market for an f350 drw that should consider the F450. In my mind the biggest drawback of the F450 is the 4.30 axle ratio and the biggest advantage is the wide track front axle. The bigger brakes of the f450 could also be a factor for those towing heavy trailers.
As far as GVWR numbers are concerned on the pickup trucks it seems as though they are not set by the manufacturers and therefore do not reflect much in the way of design limits. The truck manufacturers' axle ratings take into account the suspension and give a more accurate measure of a truck's weight carrying capacity.
Why truck manufacturers bother advertising payload capacity is beyond me. I guess boasting greatest payload beats be accused of being the lightest built truck in its class.