Regarding keyfeel and quality...I got along fine with PSR's, but I'm used to adjusting to all kinds of keybeds, from the early non-touch synths to what we have today in the TOTL arrangers.
Regarding the choice presented in the topic...
The Tyros3 uses the FSX keybed, which is also on Tyros2 and T4/T5, and besides this keyboard having a more robust feel and deeper/longer throw, it also has aftertouch, which, once you are used to it, can really add expression to your playing, especially lead lines.
The S950 keyboard lacks the above features, and although once one becomes accustomed to it and, it's quite livable, if you are fussy and want superb no compromise keyfeel, then the Tyros3 is the way to go.
The T3 also has SA2 voices, much better feeling (and more robust) pitch and modulation wheels, a better screen that tilts, three assignable pedal inputs (S950 has two), a handy set of sliders, one more USB to Device port, easier to read buttons, and will give you three right hand sounds (as opposed to S950's two).
If the Tyros3 is in mint condition, and shows no obvious wear, I'd pick it over a new S950 if I was presented with the choice.
This also may hinge on whether or not you have a decent sound system to use with the Tyros3, as unlike the S950, it does not have speakers (this can be seen as advantageous to some as they can choose their own system).
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.