def the yamaha....... besides me myself being a huge supporter of the P series..
well objectively, the touch of the P120 is so much better than the touch of the SP500. I myself would (if i had to) get the SP200 instead, because the touch is much tighter. the sounds sampled are also so much better, from a CFIII yamaha (or unoffically C9) grand, and i assure u, its a most beautiful grand. I've never played on a C9 before, but i had a C7. And already i was blown away.
yamaha's warranty/repair dept is also located here, in singapore, and its less of a bitch to repair than korg. (even though, u almost never have to bring it back)
the korg however, looks much better than the P120, unless u're buying the P120-S. That looks slightly sexier than the original P120.