MICA,
You can record the music that is being played on both these arrangers by using the sequencers etc, and "easy record" in the case of the KN6500. ...a doddle!.
You can't record your voice though on the KN6500, even though there is a mike socket on the back. The Yamaha 9000 will record your voice though I would have thought, through the "in-built" sampler - which is upgradeable to 64 MB of RAM - plenty enough for a complete song, whereas the standard "ONE" Megabyte (MB) is not.
J5