That depends on what you do with Wyll. Is he a bladelock and melee? Then I would rather go with s good armor, he uses his Cha modifier for attacks.
In the Githyanki crèche you can get gloves that give you 18 Dex, if you want to buff his armorclass more.
If he is full caster, then use the potent robe for more EB damage.
