Use the +Atk one. It's actually quite a decent IV. Lower speed makes her able to fire off Aether every fight with Warding Breath, whereas her Atk is very mediocre to start with. -spd doesn't avoid that much less doubles, although +spd would.
IMO, either +Spd-HP or +Def-HP are the optimal sets for her, with +atk-HP and +atk-spd being close behind. But you can just use the current one and merge into a better IV when you pull it. With Fae now available at 3* it won't be long.