Cassp,
I have a good friend who held on to his 3k because he felt the upgrade was of no real use to him.
He plays guitar and sings along with SMF files that he makes himself.
Because he rarely, if ever, uses RH voices for instrumentals, it was not beneficial to buy an S900.
If great RH voices are important to you, then the S900 is a terrific buy...otherwise the 3k can do the job just fine.
I still have my old 3k, although it is now for backup and is in storage, and even though it doesn't have the neat rubber buttons like the S900...it still is a fine instrument.
Ian