The upper address sag better and the lower address sway better. It is hard to say if you will have either problem, so do you really want to buy something you may not need? The lowers are easy to install if your lower spring is predrilled. If they are not and someone is offering you free labor to install them, that may be a worthwhile purchase. The upper ones are easy to install but you may have to move back a plastic fender liner in some trucks to access the overload perches.
I had to use both on my 2005 F250 to level out a 4000 lb camper. The F250 of that era sat 2" lower in the rear than the F350 but had the same suspension when optioned with the camper package.
