Bushing dies squeeze the case neck to a uniform outside diameter. The thickness of the brass determines the inside diameter. Since brass thickness varies among different manufacturers any particular diameter bushing can produce different inside diameters. The bushing die manufacturers recommend getting a specific diameter bushing to match your brass.
The Lee collet dies squeeze the neck around a mandrel or spindle to a uniform inside diameter, regardless of outside neck diameter. The mandrel Lee supplies usually gives a light bullet pull but you can spin it in a drill with some sandpaper to reduce the diameter for a stronger pull, or just buy undersize mandrels from Lee which are pretty cheap.
If you use a bushing die to reduce neck diameter too much, IME "too much" starts around .003-.004", it can produce crooked necks.
The Lee collet die produces a straight neck every time. I sort my brass by neck concentricity, such that the good brass doesn't vary in thickness by more than .001" or maybe .0015". Those cases after sizing in a collet die will show total runout of .001 to .0015", i.e. the only thing being measured is the neck thickness variation. The collet die produced .000" of additional runout.
Bushig dies are expensive, collet dies are cheap.
They are a win, win, win situation.
