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Neither.

What's kagero's build and IV's?

I need to see the IVs and Stats of Kagero.

W!Cecilia>H!Kagero

Winter Cecilia due to having (eventual) merge potential

Are you using Kagero's native weapon?

In either case I'd probably pick Cecilia.

I'd put it on W!Cecilia (already have actually) simply because her weapon is extremely well suited to being used defensively. It's kinda like a purely defensive Owltome, she can get up to +6 Def/Res when enough allies are within 2 spaces of her. Also, she will very likely be a heroic grail purchase later, which means she will become much easier to merge than H!Kagero

I'd also recommend putting CC on the new Eirika as well, if you manage to pull her, with an Absorb staff, Vengeful Fighter, and Miracle, you have one hell of a stall tank