Hi Don,
the new grand piano sound is really good. As far as other sounds, there are a few additions as well as improvements, like live vocals. One of the new voices is a "Carlos-Guitar" which is really only a Jazz Guitar with distortion. As I mentioned in an earlier post, I wasn't very impressed with the PSR-9000 when heared it the first time. When I listened to the Pro for the first time I almost felt it was a different KB. The sound seemed much more real and alive. I'm sure they tweeked some of the DSP effects. Also as Clif mentioned the Pro has 48MB vs 32MB. As to the other voices I don't have a direct comparisson, maybe our friend George knows more.
Hope this helps,
Eric