Right now, I feel Zola with that much cost reduction is better. But you should still attempt to bring Kanon to serviceable state, as her skill awakening looks pretty interesting, and feature itself should be implemented any time soon (like, yesterday, actually )
Hmmm... err... maybe when you don't quite have enough ehp to block that mithril golem? Or your tank is facing three mid-bosses and isn't armored enough to take on them all / your healing is not enough to keep up? Something like that, probably...