If you are going with the F250 and are worried about sag, then get the "camper package" that costs about $160 more. Most people don't know about this package and it gives the F250 the same 4 leaf leaf spring with the same total thickness and spring rate as the springs on the F350. It also adds a rear stabilizer bar and extra heavy duty front coils on the front like the F350 as well. Essentially it makes the truck an F350 with F250 decals.
Like I said, most don't know that Ford offers different suspension packages in their super duties so they think they are all the same.