There is lots of information here: http://www.originalz.com/casio/casio_bbs/
Look for the heading "Thinking about buying a new Casio WK-3X00/CTK-691 keyboard?"
In a nutshell, the wk3000 is the best value for money around. And it sounds really good too! But if you can, you should try it first to make sure it is right for you - since everyones requirements are different.