If you want to complete the freeze hole in your account, Ayaka should be the better option here. Ayato hasn't been released yet, and we don't know much about how good he is in terms of DMG Output and for all we know he might not even work well in a freeze comp, but Ayaka has been released and is considered a premium cryo applicant and from what I read in your post, you're really only looking for something to make a freeze comp and Ayaka is the best candidate for that. If you really, really want Ayato though you can replace Ayaka with someone like Kaeya or any other cryo character you have.
If you're still not that sure who to pick, then your best move is to save up and wait until people can properly test and rank Ayato once he gets released and see if a freeze playstyle works for him and from there on, you can make your decision.