I don't see any direct comparisons or reviews online for the cameras, but looking at the posted specs, these are the main (important) items that I see:
SD890:
5X zoom (37-185mm)
1/2.3" CCD sensor
F3.2-F5.7 aperture range
Continuous 1.2 FPS shooting
Improved face detection (auto white balance and face tracking)
Around $350-$380 U.S. new (based on a quick Google search)
SD950:
3.7X zoom (36-133mm)
1/1.7" CCD sensor
F2.8-F5.8 aperture range
Continuous 1.5 FPS shooting
Optical viewfinder
Intervalometer for "time lapse" photography (movie mode that takes a photo every 1 or 2 seconds)
Around $320-$340 U.S. new (based on a quick Google search)
Both cameras use the DIGIC III processor and the same 2.5" 230,000 pixel LCD.
Just based on these specs and without seeing any actual reviews or photo samples, my impressions are:
* Most people are never going to notice the difference between 10 and 12 megapixels unless they are printing VERY large photographs.
* The bigger sensor and faster lens on the SD950 are likely to produce better low light photos with less noise than the SD890.
* When working outside or in bright lighting, the optical viewfinder on the SD950 can be useful since you may not be able to see the LCD clearly.
* The SD890 should focus a bit faster and more accurately in portrait/party situations where human faces are involved.
* The SD850 has a longer zoom (5x compared to 3.7x) if that is important to you.
Short version:
It's hard to say for sure without seeing actual photos from the cameras, but unless you really need the extra zoom range, I don't see any reason to recommend the SD890 over the SD950.