In my view, the main differences between the S775 and S950 in terms of what they can actually do are:
+ S950 has vocal harmony, S775 does not
+ S950 has audio styles, S775 does not
+ S950 built-in speakers are a little better than S775
+ S775 has live control knobs, S950 does not
+ S775 uses the more flexible YEM system for expansion packs, S950 uses much more limited system
It's up to you but my advice would be if you don't want to sing along with vocal harmony, go for S775.