Yuna having aeons isn't really fair (so I'm excluding them)...and considering the fact that if both had the sphere grid maxed out (on my file they both do), then both Yuna and Rikku would do about the same amount of damage with holy. I would have to go with Rikku as even with the sphere grid maxed, Rikku's faster and would get a few turns in before Yuna could even take a shot at her. Especially if both had break damage/HP limit (Rikku does about 99999 each time while Yuna only does about 70000), Rikku would still kick Yuna's butt because she does more damage when hitting physically and would take out all of Yuna's HP in one or two hits before Yuna could even take her turn.
If put in a battle of personality, Rikku without a doubt.