If you can afford it, I'd suggest getting the S100. Although it may not look that much different based on the specs, the sensor is improved, as is the image processor (the DIGIC V chip). You also get a bit more zoom, and 1080p (24fps) recording on the S100.
That said, if budget is a concern, the S95 would still be a decent camera.
As luck would have it, I played around with an S100 today and I did like it overall; though it was little too small for my tastes (I had my eye set on the G12, and ended up snapping one of those up instead). I was impressed with the responsiveness and quality of the S100, and stand by my belief that it would be worth paying a bit extra for it over the S95.