There's a heck of a difference, in price but also in spec.
Most important example for me: S775 has 76 S.Art voices, but the S975 has 140 of them. Same sort of thing in all the other 'higher quality' voices (as opposed to what Yamaha call 'regular' voices.) you get more Cool, Live and Sweet voices, plus 10 more Organ Flutes. The total voice count on the 975 is 1090 compared to 930 on the 775.
Same with the styles: 523 vs 433.
For most players, styles and sounds are the prime factors when choosing a keyboard. Everything else might be a big bonus for some but an unwanted feature for others.