I have the S90ES and the piano sounds are outstanding. The MO8 doesn't have the additional piano sounds that the S90ES has included. There are other additional sounds that are included in the S90ES that aren't found in the MO8 (various electric, strings, synths, drums, appregios). You can also install the PLG boards for extra sounds on the S90ES, the MO8 doesn't have that feature. For the most part, everything else is about the same. Both sound good.