Ishtar Iv help

+speed for sure. Doubling everything is more important than 3 extra damage if she can't double. Plus she'll still have her physical bulk and be fast enough to survive a red mage and maybe a blue mage. Alternatively you could give her ardent sacrifice to get into desperation range without worrying about damage.

If you are going for a bladetome build, +Spd is the best one, but if you run Mjolnir, +Atk is not bad since she reaches absurd speed levels with it. But -Hp hurts her a lot since she is very frail, so I would go for the +Spd route.

I think I prefer +atk unless you're using a blade tome (which is really against my taste for her but objectively fully functional and effective, playstyle aesthetic aside). That is, unless you buff her spd or have merges. If you're merging to +6 or higher then consider +spd because she gets so fast she'll probably desperation moonbow through Nino+10 if Nino doesn't use a spd seal, and similarly for other fast units including even some Ayras or Karlas. You can achieve similar results with a spd buff.

Otherwise, take the extra attack. You might need it against some tanks with tons of HP