Karla vs. Ayra. My thoughts

I don't have either, but from a theoretical standpoint these two are quite interesting. Ayra has a lot of flexibility due to her insane special, but Karla is even more flexible because she doesn't even need her special if she goes with her prf, which comes with special acceleration and extra damage. The only weapon I usually see Ayra switch to is slaying edge, but Karla can take all manner of different specials, including - as featured on a very current post - AoE specials.

As you say, Ayra has a significant edge in physical bulk, but I'm not convinced that either of them can reliably take more than one hit, especially if the counterattack comes with 50+ atk. Sword Lyn is currently the only unit that can use both desperation and wrath (unless you want to go all-in on Hector's speed), so Karla has an advantage in this regard, not being special-reliant.

I do think that both Karla and Ayra are going to be common targets of chill spd, especially the Ayras I've encountered, so I don't think Ayra is doing better than Karla here.

I don't even see a contest for both of them when I read regnal astra. Scoring should be you highest priority, and with 500sp special, Ayra is just better. If it is not for arena, I could see a point for Karla being better. All in all, they are pretty much similar. Also, Ayra won't die from Bold fighter from other unit than Zelgius, as Aether has a 5 charge and won't proc, and you will mostly just initiate with these unit so taking a bit less damage can be usefull.

I have an Ayra, over rated. She's a solid unit, but she is very generic and doesn't offer much special imo. I have about 17 other 5* sword units and I prefer about 9 of them over Ayra, so she doesn't even show up in arena assault.

I have both with optimal ivs and in my opinion it's all about preferences.