Polyphony is important for two things - sequence playback if you are going to be playing lots of notes at once, AND/OR using voices that have a long decay like a glass harp or something like that. Piano and guitar don't usually cause problems, though, unless you are holding down a sustain pedal.
Polyphony is also important if you can do multi-timbral and are sequencing multiple instruments simultaneously.
I recommend you outline what you want to do with your purchase and make certain that you are getting what you want. Don't worry about older models or features you won't use. Buy what will let you express yourself. (Gawd, I sound like Madonna, now!) If you buy something that limits your creativity, you will have buyers remorse and your synth will be nothing more than a big doorstop.
[This message has been edited by digiboy (edited 07-07-2000).]