I had the S950, own the S970...
Biggest differences, and they are significant to me:
Larger screen
Active control knobs...use them all the time
Bigger on board memory
Audio styles...sound excellent live
VCM...love this feature..a person can really shape voices with it
Speaking of voices, it does have human voices...sounds pretty good
Multipads...I like the way the multipads are organized...makes using them quicker and easier.
When I had the S950, I used it for backup and took the Tyros to gigs....10 - 14 a month.
Now the S970 is my main board and the Tyros stays home most of the time.