My gf was looking for a slim camera and we ended up with the 85IS because I think the only difference was 2.5" screen on the 85IS and 3.0" screen on the 90IS. I think the resolutions for the screens were the same too, so the 90IS is bigger but the image is just stretched compared to the 85IS. The 85 is a little more rounded too, the 90 has sharper edges.
It takes great pics for a compact camera though and the face recognition works very well