i think kuja is a better villain and is more powerful but the battle could go either way. kuja can fly very quickly (he goes from terra back to gaia when he destroys terra.) and he could destroy sephiroth with magic from afar. sephiroth can jump but i'll bet kuja can fly higher. but if sephiroth gets near enough, he'll use the masamune and win. tough choice. depends how the battle works.