I;m torn on this as well. I feel that either the fight on Gagazet or the Fight Inside Sin is uneccessary. The impact from having him be sent inside sin is bigger than if you sent him at Gagazet, but I feel that the fight with him before Zanarkand is also important as it leads to the ending of the journey for one final trial before the "final dungeon".

I dunno. I feel they are both needed, but at the same time that one of them is unnecessary.