Hello Paul. Good to hear from you again
I have an S975, and have tried the SX900 several times in our store, and I also have the Genos, with the same touch screen as the SX, but the screen is bigger.
It depends on what you like. The registrations from the S975 won't work on the SX900, at least not without a lot up rework!
The SX900 has better speakers, 3 right hand voices, touch screen, and it now has scat voices previously only seen on Tyros/Genos. Some styles are reworked. I am sure the SX900 users will chime in and tell you to go ahead, better keybed, better sound and so on. Only you can decide if the new features will justify an upgrade. I strongly advice you to try it if possible. I will not upgrade, but keep my S975, but I have the Genos also, so I really had a hard time justifying an upgrade
It is worth noting that the speakers of the SX900 are different from the S975 in that they point more outward, so the sound is not only going up to the ceiling. On small gigs, you can use it without a PA system. Good luck deciding