CaS gives back almost 900 MP? That gives you a Dark Passenger or 1/2 a Dark Arts to use on Soul Eater.

Soul Eater potency with DA added is 140. Dark Passenger is 150.

Using DA on CaS nets you 350 potency.

So you're looking at roughly 70 potency on a Soul Eater from 1/2 a DA, or 150 potency from Dark Passenger (assuming 300 if you get 2 off for the same MP). A DA'd CaS is a 350 potency gain.

I don't know about you but the math seems fairly simple.