Sephiroth wants to become a God when it comes down to it. Kefka wants to become more than a God.

Sephy wants to be the one that kills the tangible and dominates the world.

Kefka wants to destroy the intangible - Hope, dreams, love... he wants them dead. No spoilers here, but Kefka is by far the most successful in his ventures.

Emperor Ghestal is all like "Lets conquore - lets rule. I wanna be an overlord pansy bitch like Sephiroth. Nya. Nya. Nya." And kefka is like "FUFUFUFUFUFUFU old timer! I'm more evil! /win"

And right about there is when I climax. The story goes a bit further, but alas, my epic stamina can only go so long.