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.

What about for stat boosting on girls that are whaleship filler? Any preference?

When it comes to whale: I would advise always go with the higher TP equips unless the stats difference is minor. This essentially means attack-based gear should be prioritized. Fillers should go more towards defense, while deck units should focus more on the attack side of things. It has been my experience that a small increase in defense can make a big difference in survivability.

When it comes to units: You really need to think about the unit itself. With that said, if it is a debuffing team, you want to front-load as much defense as you can. Only once you can surivive what I would consider 8-12 hits without healing would I focus then on the attack side of things.