They are different. Diana is mainly used for her 15% DEF down debuff, also her 15% assault party buff is not bad.
Sati on the other hand is strong offensive character against dark. 50% crit buff is no joke and her nuke has quite good cap for SR (450k). She can definitely deal some serious damage in short period of time. Unfortunately against anything other than dark she's pretty much gutted.
I'd say than in Diana is easier to use (shorter cooldowns, assault buff gives more when grid is poorly slvled), while Sati has higher potential (against dark). Personally I'd choose Sati, but this largely depends on your own situation and what you need most.