Yuffie has good dialogues with cloud throughout the game, has a challenging side quest, has useful limits, has a great ultimate weapon, and is cute/spunky/annoying in an endearing way.

Vincent has a good backstory but is lousy in battle and his limit is really annoying. Vincent is also more of pain to get into your party in the first place. Plus, he uses a gun which is dishonorable.

I'd give Yuffie a 10 and Vincent a 4, so I averaged that out to 7.