I currently have both, and am trying to figure out which to hang onto. I must be in the minority that prefers the S90 body style to the S95. The 90 looks and feels more metallic and robust than the 95. I actually like the "slick" feel of it, and I don't have a problem with the loose rear dial.
IQ-wise, the 95 has a small edge, but I prefer the slightly brighter video of the 90. The 95's HD video quality really isn't that great compared to other brands' HD modes. The S95 has a nice DR option, but it's not much different from the S90's I-Contrast setting.
They're both amazing cameras, but I don't see there is any reason to upgrade if you already have had an S90.