I own a MoXF6. As all the waveforms and voices are 100% identical to the Motif XF, it must sound better than the XS, exactly speaking the voice banks 1 to 8 sound identical, bank 9 contains the 128 new voices which are higher quality.
I don't think the 'general sound' apart from the waveform question differs in any way, after having compared official Yamaha Motif XF audio demos with the sounds I hear from my MoXF.
Here are links to the official Motif XS demo 'Memory of the King' (recorded directly from a Yamaha website), then the same demo roughly played by ear on my former MoX, then the same demo again played by ear with better samples from my MoXF (including the Garritan GPO installed on the MoXF flash board):
https://soundcloud.com/rorosetree/former-official-yamaha-motifhttps://soundcloud.com/rorosetree/yamaha-mox-former-yamaha-demohttps://soundcloud.com/rorosetree/yamaha-moxf-motif-demo-memory