OK, took delivery of the Korg Thursday and thought I'd offer up a few initial observations...
When I first got my SX900 it was an upgrade from an S950. The OS was somewhat different from the S950 which I was not expecting and turned out to be somewhat of a challenge. Sweetwater, where I bought it, offers a 30-day return policy with no questions asked. I almost sent it back, but in hindsight am very glad I kept it as it's turned out to be a great keyboard.
At the present I'm in the same place with the Korg. The much-touted keybed is as promised, very nice indeed. The touch screen is much more readable than the SX900. "Build quality" seems very nice. The keyboard has a nice look and feel to it. Now, this unit was a demo, and as such didn't have the latest software upgrade, which I performed. Sooooo, the OS is certainly different but I believe "learnable" for this player at least...but it's akin to having to learn a different language. I grope around trying to do the simplest things. The voices are on a par with the Yamaha, some better, some not. Love the drawbar organ, piano, electric piano.
Strings are great. Horns, about the same. Sax is not as good, along with the vocals. The styles are, well, disappointing as of right now. The amount of song/style editing appears to be almost unlimited, so I just have some learning to do. Song/style storage is pretty straightforward.
At any rate, I have 28 days (and counting) to make up my mind, so I'll just have to see how this shakes out. I already have an eager buyer for the Yamaha, but as I don't have the original packaging, shipping could be a challenge.