The Fords use a 2" taller rear axle block in their 350's than 250's. Most of the 3/4t and SRW 1t trucks use the same 10K lb axle and suspension and only differ by the wheel capacity. Usually you find 3/4t trucks have 6K lb RAWR and 1t have 7K lb due to the above.
If you are choosing a new tow vehicle, it best to choose the 1t unless you get penalized for excess weight fees on your registration. Those fees could go to upgraded wheels on a 3/4t to give you the same capacity. Don't overlook camper packages that will give you higher rated springs and anti-sway bars when comparing vehicles. I know GM does not offer the same wheelbase options in the 1t as the 3/4t, that could also be a reason for choosing one over the other.