Checking the specs on the Ford site, the 2016 F450, DRW, 4x4 has a max GVWR of 14,000 and a payload of 5,300 for a crew cab. This is compared to the F350 DRW, crew 4x4 which has the same max GVWR but a max payload of 6,460, both LWB trucks. Keeping to the same trucks (crew, 3x4, DRW)but moving to chassis cabs, the F350 has 6,937 for max payload, the 450 is 8,977 and the 550 is 11,800 all of which are for the 176.2" wheel base to keep things consistent as the 450 and 550 also come in a 200.2" WB.
As I have looked at Super Duty trucks, having owned 3 so far, I can't make sense out of the F450 as it has the same GVWR as the comparable F350 and a lower payload. The difference that I can find between the 450 and the 350 is in towing capacity. They are identical for conventional towing but the 450 maxes at 31,200 for a fifth wheel compared to 26,500 for an F350. Having routinely conventional towed 10,000 lbs. with my F350, I really don't think that I would want to max out the towing capacity of any truck. I have a 2013 F350, crew, diesel, crew, long bed. I love it and have had no issues with it at all and now am at about 25,000 miles. When we got the truck camper, the 2011 F250 (same truck right down to color but SRW was not sufficient and we went to the F350. Now that I am seriously considering a flatbed mounted TC with a 3 foot cross bed box on the bed forward of the TC, there is likely to be an F550, crew, diesel, 200.2"WB in our future but that is likely to be a 2017 chassis cab.