Both are great keyboards. I have owned both.
I liked the S90 pianos (the new S700 samples) very much. I liked them enought to re sample and loop them into Giga and Kontakt.
Overall the S90 wave rom is much better than the S80. Rhodes, drums, orchestral stuff is all way better.
It is lacking in pads, breathy stuff, good evolving patches.
I just put my S90 up on ebay but only after I sampled out a bunch of the waves. I liked it that much.
Hope that helps