I currently own the Canon SD870 IS, my previous P&S was a Fuji F30 and I have had extensive play time with the F50. The following is a summery of my own evaluation results before purchasing the Canon SD870.

Noise performance per ISO: Fuji F50 wins.
The F50 seem to have about a 1 f-stop advantage over the SD870. i.e. an iso400 photo from the F50 have similar amount of noise as an ISO 200 photo from the SD870. (this should not be interpreted as the Fuji generate better looking photos, see next section on pixel quality) The F50 has noticeably worse noise performance than the F30, inevitable considering the doubling of the pixel count with only marginal sensor size increase. Slightly disappointing due to the expectations build up from the F30/F31fd, but still better than most in its class. SD870 IS users should note there IS a difference in image quality between the ISO 80 and ISO 100 setting.
Pixel quality per ISO: Canon SD870 IS wins.
The SD870 tend to produce nice defined details per pixel when files viewed at 100% crop, there are pixel detail loss, but is comparable and outperform most cameras in this class. On the other hand, the F50 exhibits an exaggerated Mosaic/ water-color look even at its lowest ISO setting. This is a phenomenon also seen the the F30, due to the honey comb pixel layout of the SuperCCD and noise reduction, but has been magnified in the F50 to the point 100% crop of the photos should not be viewed on monitor. While the F50 might have less “noise” at higher ISOs, but the Mosaic effect make the image look much less crisp than the SD870′s at ISO 400 and lower.
Build Quality: Canon SD870 IS wins.
Comparing the SD870 and the F50 side by side is like comparing an Audi vs a Kia. The screws on the Canon looks tighter, the body feels more solid, seams smaller, and textures more substantial. This doesn’t mean the Fuji is not rugged, I was able to abuse the F30 as my everyday camera for 2 years carrying it everywhere without any broken parts. But the texture difference make the Canon more pleasurable to use in real life.
Software Interface: Canon SD 870 IS wins. (barely)
The 2 cameras share a very similar menu heirachy, with one quick function set and one comprehensive function set. The “Func Set” on the Canon does offers a superset of the basic adjustments under the “F” on the Fuji. The Canon give you a bigger set of the oft adjusted parameters here, including a useful collection of color response profiles. The Canon also remembers your last modified variable, so if its something you use often, its nice to not have to scroll down from the beginning. Both camera’s full menu are tiered similarly. The Canon offers slightly more in terms of managing/organizing your photos and controlling the behavior of the focus points and such.
External Interface: Canon SD 870 IS wins. (barely)
The Fuji have a dedicated exposure compensation button when in “M” mode. On the other hand, the Canon have a customizable button that can be a dedicated exposure compensation button or a number of other functions. This flexibility offered by Canon is surprisingly useful and flexible depending on the shooting situation. One minor gripe I have about the Canon concerns with its flash modes. The Canon offers the same number of flash modes as the Fuji, however, its slow-sync mode is enabled in the menu rather than cycled through the external flash mode button! This is a strange choice and incontinent for creative photography.
Photographer’s Tools: Canon SD870 IS wins.
There are 2 features that are worth mentioning that the Canon has the Fuji does not. First, AE/AF lock by the press of a button. This is a surprisingly useful function that has been requested by Fuji users on a number of forums, why have they not added this? Second, Hi-ISO shift, where upon detecting possible camera shake would light up the customizable button mentioned above, if you press the customizable button while its blinking then the camera shifts to ISO 800 to reduce blurring. The functionality enables a photographer to keep the ISO as low as possible and only shifts to higher ISO upon their own decision, it give an added sense of control to the photographer while maintaining the ease of use of auto iso shifting. It could have been even more useful if Canon’s high iso performance was better.
There are 2 modes in the Fuji that are useful on paper but useless in real life. its Aperture and Shutter Priority modes. This is fantastic on paper, mimicking SLR functionality. However, due to lack of dedicated wheel or scrolling for the function, it requires multiple button presses to change. Turning it into a hard to access function.
Lens: Canon SD870 IS wins.
The Canon’s lense starts at a 35mm equivlent of 28mm while the Fuji starts at 35mm. The additional zoom range allows for more creative shooting with the Canon. Further more, the Canon lense exhibits less corner softness. This difference is not noticable unless you expand the image for big prints.
Out of camera Image Quality: Tie. The difference is a matter of personal preference.
The Fuji tend to have more muted output in comparison to the Canon. I personal prefer the Canon output due to its slightly warmer tones and the general color space looking more pleasing to my eyes. However, this is something anyone with basic Photoshop knowledge can mimic k in batches for the Fuji.
In terms of sharpening, the Fuji does a bit more in camera sharpening, and therefore you seem more artifacts from the pictures if you pixel peep. The difference is minor and unlikely to be noticed by most people.
In terms of contrast, the Fuji seem to use a steeper contrast curve adjustment with inopportune highlight clippings. On the other hand, the Canon tend to over expose scenes with a high contrast object and over exposes the bright objects. Both are correctable through the functionality provided in the camera menu.
Image Stabilization: Canon SD870 IS wins.
Fuji uses a CCD shift type IS system. Where is it? I do not notice much of a difference at 1/30 sec, the F50 generate as many blurred images for me with IS as without IS. Canon uses a lens shift IS system that seem to work relatively well, at 1/30 with IS on, my photos tend to be sharp 90+% of the time, and most 1/15 sec images are usable as well. The guess is about a 1-2 stop effectiveness for the Canon system depending on the zoom.
Screen: Canon SD870 IS wins.
the Canon screen is big and bright at 3.0 inches. The Fuji is a 2.7 incher. They have about the same pixel count. This is a feature that matters little for me, as I prefer even an inaccurate optical view finder over the LCD framing. However, for the size queens out there, the Canon one does seem a touch brighter and more contrasty, and it can be viewed very close to the full 180 deg range.
Value: Fuji F50 Wins.
For 100 dollars less than the Canon. The Fuji compromise a little bit in terms of usability, built quality and feature. However, the difference is not worth 100 dollars to most users.
So in the end, why did I pick the Canon? While the difference in features/usability is minor in each category, there are a few features I value in the Canon — 28mm wide angle zoom range, AE/AF lock, functional image stabilization, and the Customizable function. In the end, the value offered by the Fuji seems to not worth the compromises for what I need in a camera. If your needs and proities are different, the Fuji might very well fit the bill and save you 100 dollars in the process.