I had PSR-S775 (is the same as S770) for about 8 months and then I decided for SX700 when it came to market in end of 2019.
As I can see you did check differences in specifications and also probably watched many videos about SX700/900. So, it's hard to tell you something "new", except personal reasons that moved me to switch -and if it turned out to be the right decision or not.
Let me say... for me, touchscreen was the main reason. Whatever some might say, it is way more comfortable than non-touch. Ok, if only for changing voices or styles, the difference may not be that significant... but at changing parameters, or at creating/editing styles, the difference is huge -and for that alone, it was worth for me. Yes, it depends on keyboard usage... but even if someone isn't that much into tinkering, the overall experience is much better.
3 voices for right hand... was/is additional bonus for me. Depends on music, but some (parts of) pieces just need more than two voices to sound more "like original". I would say, sound-wise, 2 vs 3 right hand voices, is the most important difference between S770 and SX700. However, how much of use it is, depends on music genre we play.
Jostick vs wheel for pitch bend/modulation.. Even I don't use it that much, joystick is much better for me. But I wouldn't call that as deciding factor, though.
400MB vs 160MB expansion memory -is also something that is probably quite important for many.
Almost forgot... panel buttons on SX700 feel better at pressing, than that was the case with S775 -it's because harder rubber is used.
Maybe I missed something, but other than mentioned above, there's nothing you can do on SX700 (and not on S770) and sound quality is also pretty much similar.
My very personal opinion: for me, keybed on SX700 feels better (for playing) than on S775 -but difference is very small. What's more important is, the keybed quality, is in long term probably better on S770. On my SX700 keys started to rattle just after a year of use. And as I later found out, I'm not the only one having this problem. Btw. SX900 has the same keybed. Solution is (at least temporary), you buy special grease, take keyboard apart and apply the grease on each key.
I hope that is of some help to you,
Bogdan