If your Kerry is level 1, it's Thetis no question. Even if you lack the silvers to CC her, her stats at 50 un-CC'd will be comparable to Kerry 50CC60 (HP she loses massively by almost 700 but Attack she loses by 8 and Defense by 22 if we assume Kerry is max affection and Thetis is the pre-CC max of 50). Naturally this means Elaine doesn't even compare.
If you're too weak to do the 2nd half of this event, CR/SU using CC'd copies of golds is a waste of time, especially given the frequency of breaks on Nutaku. Level your units, hold onto the spares and worry about possibly leveling them for CC during the likely break week following event end.