As I finally took my SX900 to a service hoping to get fixed "the noisy keyboard" problem, I had a reason to take my old PSR-3000 out of a bag and play it.
Surely, PSR-3000 doesn't have a lot of new and useful sounds. In the same time, I noticed something, which I also noticed comparing it to PSR-S950 before, the overall sound feels a bit more "joyful" and "bright". As this is very subjective, I don't plan to go deeper about it...
As for the controls, I have to say that these plastic buttons proved to be more reliable than rubber ones on PSR-S950; they feel solid, yet not at all noisy.
Basically, so far I don't see any practical advantage of a touchscreen on SX900, in fact, it makes some adjustments rather uncomfortable; maybe just for typing names it's better, but as for other things, like muting parts, it's not very good.