+Spd, +Res, or wait for a +Atk Caeda

I think Attack is Caeda's most preferred stat. She has 37 base speed, which would increase even more with Hone Fliers (and even higher than that depending on the build), meanwhile, her attack stat is at a base of 25 (neutral).

speed is really good for her weapon but her defense is actually pretty decent considering shes a flier and it would be better if you got one that was -hp instead of -def

Honestly, i think +Spd is the best for making use of her weapon, although +Atk is preferred, but with +Spd she will out speed most things you might encounter in Arena, or Arena assault which is very helpful.

Attack is easily Caeda's best boon. It's the stat she most needs to patch up, it's a superboon, it synergizes well with her weapon's effective damage, and she's already fast enough to double just about everyone with only her base speed and buffs.

If you have feathers to spare, you can promote one now and merge later. Otherwise wait for +Atk, it makes a huge difference.

If you give her DC, then run res, but if you want her to be player phase, then +spd outshines +atk imo since she is one of the few units who can potentially reach the ayra speed tier.