FTNITK, Sadako is the scary-ass chick from Ringu, and Kayako is the scary-ass chick from The Grudge.
This movie was originally just an April Fool's Day joke, then turned into the real thing, possibly because of the positive response put forth by Japanese fans.
I've seen the trailer. It's as expected. Though both of these original movies scare the shit out of me, the trailer made me laugh. A little with joy, a little with nervousness. It's a monster mash-up, and I get a great kick out of that idea. Will I watch the fuck out of it? Yep. Will it be scary? Maybe. Will I be scared? Fuck yeah. Will it be super fun? I'm hopeful.
Seems like the sky is the limit with movies these days. Anything can happen. And though they tried this before (Freddy vs. Jason, for example) this one might actually be a real horror movie, with real scary stuffs. So I'm kinda pumped.
Short Answer: If this is asking who wins, gotta go with Sadako because she's evil as shit. Kayako was always more of a sympathetic kind of murder-ghost in my esteem.