Mia: +spd -hp or +atk -def?

I can make a +10 Mia. I have two Mias with decent IV, one with +Spd -HP and another with +Atk -Def. She will have summoner's support, since I don't have any other good candidate for it.
I think an attack boon is better, because, without a boon, she would have 46 spd at lvl 40 +10 with S rank support, she would double almost any character in the game, her flashing blade would probably always trigger, and it'd almost impossible for Mia to get doubled herself under normal circunstances.
However, everywhere I look I see people saying that a speed boon is better, and a HP bane might be better than a Def bane. Which should I choose?
Any build work, she has most skills that an offensive unit can learn, including LoD, DC, Fury, Wrath, Desperation, Atk Smoke, Threathen Spd, Aether, Luna, Moonbow, Glimmer, etc.
If you want, you may suggest team members as well...