personally, and just me. i have worked hard on my fundamentals
i want to keep everything the same in my swing whether its a strike ball or spare ball.
same loose shoulder, same pace, same tempo.
you want your body to retain muscle memory no matter what you are throwing at, so you would want the same weight spare ball (ideally)
IMO (only mine) my timing is important to my targeting that i don't want to change anything.
not to say you or anybody else cant use a lighter or heavier ball, that just my thinking