I think the key feel on the 900 is wonderful - is it the same on the 600? Anyone know?
Aside from that, the 900 gives you 2 media players and MP3 recording. Not sure about the 600, but I enjoy both of those features.
I think the mp3 and media players are on the Korg PA 600 too. I haven't played on the 900 so I don't about the keys...