Well, on the 1/2 ton rear springs, the leafs and coils are both rated the same. If you are loading by the amount that the truck is designed to haul, then it does not matter which you have, they will handle it the same.
If you go a little heavier than rated, the leafs are better.
If you load to the stupid level, (like 2500 pounds in the back) then the coils will be better. They will go into coil bind and stack up on each other making a suspention lock out and you can haul like that.
not that I recomend it though)
I wonder what it will take my one ton longorn to squat...it has the overload springs on top of the one ton longhorn springs.