I don't know. I've only played a little bit of FFX, so I don't know the characters and their abilities very well. I never met Yuna when I played it. I stopped when I was still in the beginning and never played since.
Yuna is a White Mage/Summoner and Aerith is most like a White Mage but can easily get other powers so I said Aerith due to her ultimate gorgeousness.
I want to fight Yuna because I like Aeris more, but if you are talking about their strengths, then Aeris because she is just a white mage and has low physical defence. :)
They both just sit there and heal people. Yuna is just slightly more in favor because instead of "I'm gonna sit right out in the open where I can easily get assasinated while performing the only spell that can save our planet" its "I'm gonna try to send you to the Farplane to save our planet and when that doesn't work I'll fight you to the death." Personally, I think that Yuffie and Rikku are much better female characters. And not just for their outfits :D , but that they can actually fight.
i say yuna cause she can do some black magic from lulu but she always has aeons so she can PWN aerith
Um well. Yuna is a summoner, but Aerith is a white mage..she can heal herself..and yeah she has that Limit Break that can make her inmortal I think?
I think that Yuna would win. Aeris was weak and didn't have much power even with her spells, whereas Yuna had powerful summons. Imagine Aeris taking on Bahamut or even Anima with her little twig of a staff.