well i don't consider kira as a veteran...there's no character in seed has been called a veteran, only an ace. both kira and shinn are not ace pilots...to be called an ace, one has to enlisted into the military for years and has capable of piloting any sorts in ms, whether advance or obsolete. and there was no mention of them both piloting ms...only gundams.
kira wins. why? it's pretty simple..."plot" armor.
i say canard can beat them both easily. why? canard is an ace super coordinator who doesn't even use seed mode in battle. plus, he defeated the most powerful spatial awareness person in seed.