Originally Posted by Unregistered

I generally thought atk was a better stat than def.

It depends a lot what girls you equip them on and what they're fighting.
For most girls, and particularly girls with skills in the offensive segment (like trigger-rate passive and lifesteal special), ATK is the better stat, especially if you like to clear maps fast.
But, for some girls, like those with the counter passive and girls with the ATK debuff passive, DEF is a more desirable stat, especially against raid bosses these girls can perform extremely well.