I'm definitely voting for Diamond here. In addition to usually preferring to root for the good guys anyway (and Diamond IS one of the good guys now), Cozy is MAJORLY over-rated as a villain. She is quite literally just a nastier, but more subtle Pegasus version of the OLD Diamond Tiara. The only reason she got as far as she did was because literally ALL the good guy characters got heavily saddled with a serious case of Idiot Ball. If the heroes were allowed to be as intelligent and competent as they should be, Cozy would have been exposed and defeated LONG before she had any chance to put any major plans into action.
Cozy's chief weapon is manipulation and Diamond should NOT be stupid enough to fall for her lies.