It's between him and Caeda for the red spot on my arena core, and intuitively I just don't really like him for some reason. Maybe because he can't use flier buffs, maybe because I like the effect on Wing Sword, maybe even just because I like Leigh's voice? Anyways, here's my comparison; I'd appreciate an outside perspective. Maybe something I'm not seeing, maybe some insight into why I'm leaning the way I am, maybe some other idea for a build for M.Corrin or for C/S skills for Caeda.
The builds I have in mind are:
Corrin: moonbow, steady breath, axebreaker (only reliable option vs Raven and against some Hectors), drive spd, deflect magic/QR/other buff. In no case do I intend to give him DC due to the other two members of the core being Nowi and Felicia.
Caeda: moonbow, fury, desperation, def ploy (switch to goad fliers when on a flier team), spd ploy
Caeda on arena def: glacies, fury, wings of mercy or desperation, goad fliers (so yes she'll be on a flier team), quickened pulse
Stats: quite similar, but I like Corrin's bulk better
Caeda: +atk -HP gives 36HP, 45/37/24/34 with weapon only.
M.Corrin: +spd -HP gives 42HP, 50/37/30/26 with weapon only, effect refine (I definitely prefer 16 stats on an ally to 3-4 on him)
However, if I use the build I'm proposing, I probably use +atk (which would justify deflect magic)
The team expects to have hone atk and panic ploy support for either sword, and likely a spd buff also.
My arena core isn't fliers, so that doesn't give Caeda that much of an advantage here. Also, I don't care for the obsession over bonus units that the upcoming arena change will encourage - that's not how I believe the game is to be played, regardless of scoring. I'll give up some orbs, feathers, and prestige if that's what it will cost. So I'm not giving Corrin any preference for that.
Conclusion:
I'm not excited about max-merging a unit just to give him axebreaker. That's not what I want to do with my +10s. I want them to compete in stats, especially in PvE. That build I gave Corrin there, doesn't it work just fine without merges? Then I don't want to throw 200k feathers away on him; I'll promote one copy and call it done or do a 4* merge. He has a better matchup against Myrrh, but Caeda should be able to take the disgusting creature in two rounds. Myrrh isn't getting anywhere against her 37 res. My 4th merge project is likely Camilla and my 5th is either Cordelia or Shigure, so Caeda has this in her favour.