[sirndpt]

if it makes any sense at all, how i understand (understood) it:

a setup sweeper refers to a sweeper that carries a setup move / that CAN potentially set up (with a boosting move, substitute, w/e) - for example dd gyarados, qd volcarona, sd terrakion, etc, in general

while a set-up sweeper means a sweeper that has already set up during a battle - for example, a gyarados at +1/+1


edit: i guess therefore the (probably very very general but, well, in my experience) rule is that the correct one to use is almost always 'setup sweeper' -- unless you're actually talking about the mon actually being fully set-up / boosted. say, idk, something like 'unaware quagsire can switch into a variety of set-up sweepers with impunity' (stfu no one get on my case for the actual veracity of this)

edit 2: might it help / be clearer to think of 'set-up' as a synonym for 'boosted' -- but more general? as in it'd cover setting up with substitute, and probably some other things i don't know / can't think of off the top of my head rn (rain dance kingdra / sunny day venusaur?)