I bought a S90 and a GRD III the same year, and ended up using the GRD III much more due to the feel of the camera and the surprisingly distinctive output. The S90/95 are just too slippery to feel good in the hand. The GRD III's 'pear skin' finish and rubber grip make it a joy to hold and shoot. The S90's dial is slippery, too.
Nevertheless, the S90's video is far, far better than the GRD III's, and the zoom makes it more versatile. When the Panasonic LX3 came out with a 24mm lens, the raws revealed a lot of distortion at the wide end which was being corrected in-camera. The S90 had a bit of this as well, although not as pronounced. I wonder how much distortion the S100 will have now that it goes to 24mm as well.
This upgrade appeals to me a bit more than the Fuji X10 due to the much smaller size. I just hope that the handling of the camera is much better than the S90. Not sure how much that little grip will affect things, but we shall see.