ATK+/RES- or SPD+/DEF- for Y!Tiki?

spd+ -def. I would not want to ruin some of her mage matchups and also patching her subpar 30 speed would be helpful to increase mixed phase survivability.

I feel like -def is easier to compensate for (ex: steady breath, def refine on weapon) but speed is harder to find. +spd and lightning breath refine for speed puts her at 36, which compensates for slightly lower def because she'll be hard to double. If you're going breath of fog, either IV is reasonable because +atk and effect refine will score her a bunch of OHKOs (I calculated this in response to another Y.Tiki question a few days ago; you can search) but +spd will net you some 2HKOs, including against slow Nowi or even fast Nowi if you give her team support.