I have PSRA2000 that is the same as S750 + BK5 + PA600...

mmm... the yamaha sound is more easy to get right out of the box.

Roland sounds not so good to my ears. maybe it's because the speakers.

Korg has good sounds, but more hard to play and listen than yamaha. The OS allow a lot more than yamaha (synthesis, FX, style editing, etc...) But more complicated

Finnaly, I am using the PSR more, because it is easy to get a decent sound

If I had to select only one in a desert island, I would go for the korg, because can do more things

But for instant playing, it's PSR