Ishtar IVs

I would go with neutral, because spd is important for her. An additional 3 atk pts doesn't mean much when you're going to double and kill the enemy anyway. Not only that, but you can easily get an additional 3 atk from regular buffs, in combat buffs, and sacred seals. You can say that getting an additional 3 spd is just as easy, but you need to keep in mind that having spd for her is more important as a stat than having an additional 3 atk, especially when her whole thing is to be a doubling monster.