I have both the Pa4X and Sx-900. I played out with both and find them, as John says, different animals. Technologically, I think we all know the Korg wins, if one is inclined to dig deep. Aside from that, I like both for different reasons.

I like my SX for the Yamaha for the eight registration button, really nice styles, which are freely available, The softer sound for smaller venues, lighter weight, and good speakers(when necessary).

My Korg is usually favored for larger venues, as it does it all in a more robust manner. I do styles, but also play over midi tracks, sing over CDG, and use my harmonica wind controller(DM48). The korg does these better.

I like both of them a lot.