Have you gone to the phoenix cave/gotten Locke yet?
This is crucial because the esper recieved teaches one of the best supporting spells of the game, Life 3. Once casted on someone, once the person who recieved the spell dies s/he will instantaneously (sp?) come back to life, making the Ultimajpointless cuz everyone will come back to life.
Also, casting berserk on the guy makes this battle a thousand times easier cuz he is a sissy when it comes to attacking

I prefer the esper cuz the paladin shield always seemed to annoying to get and using the esper allows you to learn Ultima earlier in the game which can be helpful, however its mostly just preference