The S950 has a few more SA voices than the S910, and no SA2 voices (as on Tyros4).

Some feel the S950's audio based drums are pretty cool, but personally, I think they are a bit gimmicky...plus there are only the 25 styles and some may not be useful to you.

If I was buying between the two, I'd get the S910 in a heartbeat, so if you don't want to keep it, there are always the Korg and Roland options...maybe the Korg SA-900 as suggested above.

The new S750 is basically an S910 minus the harmonizer/mic input. It has the SA voices and the same 128 poly as well. Better on-board speakers/amps and a black cabinet, but the panel buttons are a bit harder to see.

Good luck.

Ian