Actually, personally, I wouldn't suggest either. I never quite understand how people who own super expensive dslr's want to cut corners on cost when buying a compact (which costs significantly, significantly less to being with), but...
The s90 was Canon's first model of this kind of camera. The body is slick kind of like a bar of soap. A number of people complained that they would hit the wheel on the back of the camera by accident and changing the exposure compensation, ruining all their shots after that until they figured it out. It's also a rather slow camera (not that the later models are exactly speedy, but the s95 was particularly slow). When I moved to the s95 the picture quality only improved a little, but it was just a nicer camera to use.
The s95 didn't have any major flaws, and you can buy a refurbished one from the official Canon store for $280:
http://shop.usa.canon.com …uct_10051_10051_276388_-1
The sd4000 was also Canon's first model in it's category. It had a bit of a reputation for having a soft lens and kind of soft pics. Otherwise it was ok.
However, it's replacement - the 500hs - only costs $180 *new* on amazon. It's main drawback is that it's kind of slippery as well, and it's touchscreen controls have gotten not-so-great reviews. But it takes sharper pics than the sd4000.
I own the Canon s100. While there has not been a huge difference in picture quality between it and my previous s95, I really do like it's very improved ergonomics, and it's slightly better low light performance (and the s100 is much better at iso3200 and iso6400 than the s95, but I rarely go that high).
To me, it would be between the s95 and the s100. There's nothing horribly wrong with the s95 that the s100 fixes, but for me it's worth the cost difference for the improved ergonomics and slightly better low light performance of the s100.
P.S. The s95 supports CHDK, while the s100 does not (not yet, at least). CHDK (I believe) lets you do intervalometer stuff, and well as a whole bunch of other customizations.