Which one is better?
I actually think Merton is the best magic in the game.
Sure, Ultima does deal some heavy damage to all opponents, but Merton does too, and you get healed from it if you have the Flame equipment.
Itīs just like Doomsday in FF9; equip absorbing stuff and you get healed instead.
I think this is a pretty even "battle", so start voting!