I'd save them as a Safety Net for SoP, then if all goes well at the end of that you can then do Abyss. Cause those SoP Rewards are Insanely Valuable even if you don't do Every Fight and Side Objective, and Potentially don't Require Units but the Safety Net is always nice. Unless you already have a bunch of Potions/Revives/Free Crystals saved up that can be used in SoP if needed in which case go for Abyss.
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Sooooo .... How bout that new Diablo !