Everyone’s needs are different. As S670 came out after S950 had been released, I didn’t have an option to consider it when I was buying S950 to replace my PSR-3000. S950 provided me with, to my taste, a richer piano sound, which is a basic to me, much more authentic nylon guitar sound, which became my second basic, and all the styles started to sound much deeper due to new strings pads, drum kits, and a more solid bass guitar.
I find Suitcase Piano on s950 much more usable that the one on 3000, although it may seem similar at first. s950 is more convincing when it comes to dance music because it has more synth sounds and drum kits.
While the number of expansion pack for s950 is limited, some of them are really usable, a “Christmas and Church” pack adds realistic solo violin voices, and a decent choir pads. “Eurodance” is a rather sophisticated pack as well, it brings a different character. Now I keep an “Entertainer” pack because it has a different piano voice and with some styles it gives an impression of a different keyboard altogether. Non of that is possible with PSR-3000.
I tried s670 at a store and it seemed to be much more closer to s950 than to PSR-3000.
To me a keyboard is like a car; you expect it to let you drive it with no extra efforts, because you have to focus on the road. In my case I need a keyboard to be a sufficient stand-alone unit when I want to play it and I use PC when I need to make some advanced recordings.
Back than I had an option to get PSR-s950 for a reasonable price. Today s970/s975 are rather expensive in Russia, but a new S670 costs a reasonable $660 even here, and in case if PSR-3000 has become too familiar, S670 seems like a proper solution to me.
As for a mysterious PSR-SX900, so far I’d give it a year before it is available for purchase. Eventually we’ll see a replacement for s670; I wouldn’t be surprised if it gets a touch screen as well, because we live in a touch screen era.
I mention it before, back then there was a successful entry level arranger Yamaha PSR-550. They kept producing it for a long time, but then all the successors were rather controversial (S500, S550, S650) because of their simplified user interface. Only S670 got more efficient controls and 128 voices of polyphony, the only obvious (intentional?) limitation it has is a cursor controls, but with a direct access button it doesn’t look fatal.
For outside use S670 is really tempting because I find S950 a bit too heavy; plus, the cheaper a keyboard is the lesser you worry about all sorts of risks.