Many differences!

PA500:
-low quality outputs; low output level
-smaller rom size
-no harmonizer
-no usb for a Flash Drive, only for midi
-poor build quality
-no MP3 capability
-80 voice poly

Here is a direct link to the Korg comparisson page:
http://korgpa.com/pa_root/en/products/pa2x_comp.html?en

[This message has been edited by hitman (edited 04-01-2009).]