Okay,
I think I read this on the Roland site...the session expansion board has the most "professional" sounds on it...including piano. The Piano expansion board was made when the xp's first came out...so it's one of the oldest boards...since then they made the xp 60 and 80, and have improved piano sounds that come with it, and that kind of makes the piano expansion board obsolete. This is what I read on the Roland site anyway.

-Tracey