Both are -HP. Speed would help her unique Wing Sword, but she could use the attack too. At a loss on which way to go.
Edit: I'm open to any build suggestions if you have them, as well. I'll be starting a second flier team, and aside from a blade Summer Corrin, don't have anyone else to go with her yet.
Atk is a superboon for her, and with the low base Atk that she has, it helps her a lot. If you don't plan on maxing her out, that's probably her best option. However, in the grand scheme of things, if you're talking about absolute max potential (ie. +10, refine, heavy SI, ally/summoner support), +Spd comes out on top. But they're both fantastic choices for her, you can't really go wrong.
I personally chose +Atk/-HP and with weapon refine, she gets her HP back as well as use flashing blade, something fliers cannot inherit on their A-skill. As such, she should make the most out of her weapon by dealing respectable damage because she's so speedy, she just needs to dish out damage. And flashing blade only requires you to have 1 spd over her target to activate the cdr. I also run fury+desperation and if I choose a seal, spd or atk can work too though I prefer spd seal but you definitely want her to have the atk super boon for IVs. With this set, you can have 43 base spd before merges and buffs, which is plenty serviceable with what her weapon does.
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