A body die just resizes the body of the case, not the neck. It's primarily for use in conjunction with a neck-sizing die, for when cases get a little too tight to chamber easily anymore. The body dies I have don't decap; dunno if anybody makes any that do.
The Lee collett die has steel "fingers" that squeeze the neck around a mandrel, sizing it down enough to hold bullets. They normally result in very straight ammo.
Bushing dies do essentially the same thing, but with a separate round bushing that sizes the neck down. But unlike the collett die, for the best results with a particular brand (or manufacturing lot) of brass, you need the correct diameter bushing to size the necks down just enough to hold bullets firmly. Size the necks smaller than needed and they don't seat as straightly.
With collett dies it doesn't matter what the neck thickness is, since they mash any of 'em around the same-sizer mandrel.
