It probably depend who you prefer to play with because imo they are worth both
Thing is... void is already really good at void stuff at sig 1. Cap iw needs his max sig to reallt shine, until then his abilities are too unreliable.But, and mind you this is just my opinion, a max sig cap iw with the right team members can do anything that void can, but most of it better with way more damage and even more utility.If you have a bunch of sig stones to feed your champ of choice, you might want to go with cap.You won't make a mistake with either of them, though.Void < cap iw < cap iw (awakened) < void (awakened) < void (max sig) < cap iw (max sig) imo.