Hi seanbaker,
First I would like to welcome you to our "family" , You won`t find a nicer group of people anywhere
Now to your question, I own the PSR-2000 , it is an arranger KB not a workstation `per-say. I think the 9000-Pro would be the way to go ,IMO, because you can do "arranger things" like Acom & Splits however it does not have an appreggiator, so I think it would be hard to simulate some instruments [ie: guitar]. The "polyiphany" is 128 notes which gives you room to work and you get the "Yamaha-sound" which is very good (you need speakers with the 9000Pro) The "Psr-550" is a very good KB but poly. 32.
I have played the "Karma" and I liked some things like the appreggiator and "build-quality" but it is not an arranger it is a "workstation - only" . It realy comes down to "live perform or studio work" IMO
An "appreggiator" lets you do things like "strumming" and if your playing a chord, {ie: C -chord, it would play C-E-G / in any way you program it --ie: C-chord , E-G-C}
I hope this helps in some way , and best of luck , also please let us know how you made out with your KB quest
jedi