If you personally only like +Def because it’s a superboon for Arena, keep in mind her BST bin doesn’t change so she won’t score higher with it. I’d keep +Def anyway since I prefer this Edelgard to be more defensive. I went with +Res. We also don’t know how common the Fruits are gonna be, so you may judge after the update and the details are out. I’d recommend spending them on Grails units or 5* units you use/like a lot but have clearly suboptimal boons/IVs.
She has follow-ups on both phases against any enemy that doesn’t deny them (if she’s healthy enough), unlike the Legendary alt, so she also doesn’t need to one-shot that much, especially if one wants to use Galeforce with her.