KAOSS is only on the PA4X, starting with OS2.0. It can not be added to the 3x. I had the S970 and PA3X at the same time and the Korg is far more advanced (my opinion of course). I can't think of much that the 970 does better than the 3X. It is not even smaller, though a couple pounds lighter in weight.
Short list of pluses for the 3X:
Wonderful vocal harmonizer
MANY programmable sliders, buttons, etc.
Songbook feature.
Conventional power cable (no wall wart)
XLR mic input
Multiple outputs.
Joy stick as opposed to wheels.
Touch Strip
Ability to use inexpensive FC5 foot controller, plus pedal, plus switch.
Chord Sequencer
Dual Music/Sequence Player
Full size keys and far better key feel.
Periodic new styles and sounds FREE from Korg.
Choice of 61 or 76 notes.
Phantom Power if needed.
There are many more, but it isn't really fair comparing top of the line Korg to middle of line Yamaha. The 970 has some great styles and sounds.
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DonM