Here's a few other things the 900 has over the 600.

Semi Weighted keys, aftertouch, the ability to use an sd card in the rear just like a usb thumb drive but having it inside where folks can't get to it.

The 600 has the sd slot but only used as a unlock port for the international styles and sounds Korg sells on their websites.

The 900 has the dual song player with crossfade the 600 has one song player.

There are additional knobs which are programmable on the 900 and the 900 has dedicated style category buttons on the front panel and on the 600 you have to call up the categories on the touch screen only.

The 900 has a built in video out jack and you cannot do this on a 600.

The speakers are20 watts each and there are 2 woofers and 2 tweeters on the 900 and the 600 has 15watts per side and no tweeters. There are more sounds and styles on the 900.

These are just a few of the differences.