I'm leaning more to +Atk since he has good Res compared to his other variants and could tank magic better. Not to mention his Res wave grants him even more Res. And it also works on melee.
Use the one with +atk.
+def is a superboon which could have potentially pushed his BST to 175, but unfortunately -res is the only superbane meaning he would still have 174 BST.
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