What Hardin?

Atk since you'll be using his bold fighter anyway.

Mostly depends on the build. If you're going for a defensive build, then a defense boon is probably preferred, but if you're going for an offensive build, then an attack boon is probably better.

If you ever plan to fully optimize your Arena scoring, then neither IVs work because you are dropping his BST from 175 -> 174 with the HP superbane. Ideally, you would take a bane in Def or Res, despite both stats being absolutely useful to him, if you want to retain his 175 BST.

But, for more general purposes, I think that +Atk / -HP is going to be the better IV spread of the two. Since he is able to double with Bold Fighter regardless of his Spd, he should primarily focus on eliminating his enemies in one round of combat.