Since the defensive stats make such a superficial difference to the damage received in this game, Evasion is way better. My party is almost constantly outfitted with two Mystiles (50% Evade, 60% Magic Evade) and the Aegis Armlet (15% Evade, 50% Magic Evade) whenever possible. The latter is less effective and also has fewer Materia slots, so I give it to the party member with the most HP.

You will utterly rape the Battle Square with a Mystile.