What is Void?
Void is one of the best armors in the game. The Magic Set is kinda lacking and will only be better than Mystic at high levels and never Ahrims; though it does offer pretty decent prayer... But the Melee set is good and the Ranged Set is BiS ranged damage, partly because it is the only armor with ranged damage.
What is a halo/decorative armour?
The defensive bonuses of the halo are similar to those of a black full helm, excluding negative attack bonuses. In addition, the halo gives a prayer bonus while the full helm does not. The decorative armour is a good option for 1 defence pures as it provides good defence bonuses