I believe Kayako has a higher body count than Sadako, because for the tape to have spread so far she must have lost a lot of her potential victims. Kayako don't have time for that and kills and curses anybody and anybody that anybody knew. Plus she's got Toshio. Plus her motivation is revenge for a cruel injustice, I mean as far as motivations for monsters can go I feel more sympathy for her than the little girl who just liked to cause pain.
I will have to watch this now and am stoked that this is a thing. But my money is on Kayako all the way.