>>58484549both cards effectively increase the damage output you can do in a single turn. there is zero functional difference between using greninja's ability on the active and simply doing 20 extra damage with an attack, as there's no way for the opponent to respond to ituntil their turn.
aegislash boosts your output by an additional 10, however greninja can snipe the bench for cyrus plays, kill retreated babies for a free point, or just poke their next attacker on the bench because your attacker is already going to KO their active, which are all things aegislash cannot do.
simply put, nearly any deck that would consider running aegislash would be better off with greninja.
