Disklavier

I've searched the archives for threads on the differences between 
the "regular" and PRO models.  Can't find much.  The only PRO that I 
played was a S6 which sounded much better!  Wonder why?  Anyway, can 
someone tell me if there is a significant difference between these 
models?  I'm going to use the piano to record (myself) as well as 
use for composing.  Thank you!  SJ