I own a music store in California and I am a Gem dealer, Yamaha, Korg, Solton, and Roland. I wish I could know exactly what you want to know as a comparison, but I will do my best comparing the major differences.
One of the biggest differences is in the way you can find and play songs and styles on the Yamaha vs the WK6. You must load them into the WK6 and you can just "point" to them on the disk or hard drive on the PSR9000 and play them without actually spending the time to load them.
I think the styles on the WK6 are better generally, but the sounds are better on the PSR9000. The sequencer on the WK6 in my opinion is the best in the industry. 32 tracks with complete editing and a large 250,000 event memory makes the WK6 hard to beat. The Yamaha sequencer is 16 tracks and has very little editing compared to the WK. I love the digital mixer on the WK6 with seperate faders for each track. The Yamaha uses up and down buttons to do the same thing and is not capable of overdubbing volume changes like the Wk6 can using the faders after you record a song in real time. (By the way, most of the customers I talk to who purchase WK 6's from other dealers don't know the WK can do as much as it can do. This is a real shame). The effects are better and there are more of them on the Yamaha keyboard, and although you can load samples into the WK, you can't sample directly into it like the Yamaha. There is no draw bar organ on the WK series like on the Yamaha and Solton X1. The WK is the only keyboard that can show music on the screen not just lyrics.
In 1997 and 1998, the Gem WK4 and later the WK6 and WK8 were my best selling high end arranger keyboards. Then in May of 1999 the Solton X1 came along and knocked the WK series out of first place. Earlier this year the PSR 9000 came along, but has stilled not knocked the X1 off it's throne. In my store, the X1 is still regarded as the best sounding arranger keyboard, and I know not everyone will agree with me but I think it is the easiest to move around on. Once you've spent a few "quality" hours with someone who really understands it, it is so much more intuitive to operate. Again, these are just my opinions.
If there is anything you would especially like to know about either the Yamaha or the Generalmusic models, please let me know.
George Kaye
Kaye's Music Scene
Reseda, Ca.