Karla or Ayra?

Maybe Ayra, her special really hurts. But it's very close.
They're both a bit better than Mia though.

It all comes down to who you were able to pull, and who you like more.

Moonbow + Vassal blade's +7 (phantom speed seal was made for her) is just about as good as Regnal Astra. Karla's stats are way better than Ayra's too. I'd give it to Karla

While I do love them both I'm a bit biased since I have heavily invested ayra, it's just ayra's bulk,weapon and her own personal special makes her more unique.

Ayra is better, imo.

She has surprisingly good bulk on top of her unique special, which means she'll survive more unfavorable match ups (really just blue mages and dragons to watch out for) and her impressive damage comes from a quick special, so it's reliable and very bursty.

Karla has poor bulk, on the other hand, her HP being really the only thing that'll keep her healthy against any non-green unit with a high attack stat. Her damage output will also be impressive but it'll be more sustained, in that it'll come from the +7s she'll be able to get against a lot of much slower units, but that requires more time to build up and will be much lower against any other offensively built unit. Ayra can get a +20 bonus with her special, or nearly that at least, so to match Karla would have to get off 3 attacks before Ayra got another special (and that is not including Karla's special nor Ayra's normal attacks).

Also speed is likely the most important stat for non-wall units, and even for them it helps more then you might think, but it does kind of have a soft-cap. Like, once you get to a certain point, increasing your speed further doesn't help anymore. Once you can get to the point you'll be doubling nearly everyone you don't really need anymore, and for Karla she can't get above the +7 anyways, and with Phantom Speed that'll be super easy to hit. Where as Ayra's special will continue to scale on it, so Karla having more base speed isn't saying much, imo, just because both of them are still in the higher end of the spectrum already.

So I think the damage output will be close, but Ayra's is more reliable in that it's a full burst and she has more bulk then Karla does.

the difference is that swift sparrow or hell even fury Karla can one-round steady breath QR zelgius while ayra loses. Karla tops Ayra

Karla is much better. Better offensive stat spread.

As long as arena is concerned Ayra is better, if only for her special! Saying people prefer to use Slaying edge on Ayra though is not real however, as this weapon is really only better on defense team... If you play Ayra, Ayra's Blade is a fantastic legendary weapon, and with the +3 speed, I'd say it is on par with Karla's. Karla's great, but Ayra still slightly edge her imo.

I can double most Ayras