this may be my own experience, but it seem the end rewards for each survival do depends on what cluster (SS, S, A, B or C) which you cleared to get points for that difficulty. Anyway to short 1t up, if you cleared SS + S + A (to get more points, the max is 180 points) you get better rewards then when you cleared any combination minus the SS. Still, this is my experience doing survival, others may experience differently from me.