Sunster, the s910 is a big step above the 900, I've owned both. I now have an s950 and feel the purchase was justified because the inference in price for me was only $400 USD. Would I have stepped up for $600-1000? I doubt it, in fact I wouldn't have. As Gary says, the differences in new editions of the same keyboard are few and the cost may not match the results. In the case of the s900 vs s910, I would say the jump would be worth it. The improvements in features in the s950 seem a good step too, BUT not in actual sound.
I would suggest that unless you just have to upgrade, your sound will not change much at all. If you're not into having the latest and greatest arranger consider an s910 in good shape. The s950 is a challenge to me right now because I haven't the time to dedicate to it what I should. I find the out of the box sound needs your personal touch, but it sounds much bigger (does that makes sense?) than expected.