Cas or RMC with Cyrene

I been using Cas with Mydei for so long and I have Cyrene(and Hyacine)now which is great. People say RMC is better than Cas but I still think Cas and Mydei have a lot more synergy with the whole HP drain mechanics. Now I wonder is it better to have RMC for charging Cyrene and giving mydei more GBG+ or is it better to have Cas for her personal damage and her drain mechanics which help Mydei build charges. Not sure which one is better for overall team damage, my vote is Cas but there's to much going on to be sure.