I have my SX600 for almost two weeks and really love it. I just play a friend's PSR S910 yesterday (many hours) for a friend party.
This one sounds no good compared to the SX600. I have not tried the S970 so I could not tell its voice quality.
What does the SX600 miss ?
1) direct access to a folder containing user data on USB thumb drives (styles, songs), my Tyros3 has this feature. It is pretty quick to go to a desired folder on the USB thumb drive.
2) lack off buttons at left and right of the screen allowing to select items shown with one button push, SX600 uses arrow keys to navigate to desired item, then ENTER to select it.
These two "omitted" features make changing styles slower during a live show.
I work around by putting about 16 frequently used styles in the USER area for quick access. Hundreds of other styles are still under various folders in USB drive.
For commonly used voices, I save them in 2 registration banks. A single button push will select the voice I saved.
3) the screen is smaller, but it is crisp and clear. I can read it easily with my old eyes. If I can, anyone can.
On the positive side, SX600 has a few features not seen on other keyboards:
1) USB audio interface, I found that this is a great feature. It get rid of my Lambda audio interface box, my MacKee mixer (for microphone). Just need one single USB cable between PC and keyboard. I could not do this with my Tyros3.
2) UNISON, a good feature during live play. I post on youtube a 2 minutes video demonstrating just this feature very clearly.
https://youtu.be/wXGDCSbnloo3) ACCENT, have not tried it yet. However, the knob#2 was configured to allow reducing the number of instruments involved in the accompaniment part.
At maximum, all instruments in the accomp part play at their set volume. Turning this knob left slowly, one by one get muted.
At minimum, only drum and bass play. Very useful feature during LIVE play.
If you can play accompaniment with right hand (using guitar or piano voices), the whole thing will sound more lively and not so repetitive.
4) right hand volume control, assigned to the knob#1. Very handy to adjust the right hand volume to balance with the style volume DURING LIVE PLAY
5) TEMPO reset button: this button exists on Korg keyboard even from the very first one (PA-50), only exist on SX serie now. I use it mainly to re-synchornize with a singer show sing out of beat !!! Just push this button when they sing at the first beat of the next measure. HATE to run into this but not all friends can sing to the beat properly.
6) good sounding voices, and .... of course LIGHT WEIGHT and LOW PRICE are very attractive.
More over, I like the audio recording of this SX600, directly to the thumb drive. On the Tyros3, recording goes to the hard drive first, then you can optionally record vocal on second track. Finally mix them then export the mix as WAV file on USB thumb drive. Too many steps are needed on Tyros3.
Recording to Audacity using audio interface of the SX600 streams line this process and not limited to only 2 tracks.