I wouldn't replace either. I'd just add in Fujin, Raijin, Kuja Trance and Seifer. Also, two bonus matches of:

Sephiroth and Kuja VS. Sora (alone and with Donald and Goofy)

Fujin, Raijin and Seifer VS. Sora (alone and with Donald and Goofy)

Can you imagine Sephy and Kuja VS. Sora!? Magic and bus-sized nodachi swings and one 14 year old boy, battling against all odds... OH YEAH!