at dpreview.com, they can do a
side-by-side comparison of the two cameras
Here are some things I noticed in favor of each:
Canon SD1000
Higher ISO range (although iso1600 on an ultra compact is practically useless for
nice pics)
More focus points (9 vs 5)
Longer minimum shutter speed (for those long shutter pics)
Faster continuous shooting (1.7fps vs 1.1fps)
Records more frames/sec in video mode (60fps vs 30fps)
2x more pixels on LCD display
A tiny bit smaller
Sony W55
Rated 4.5/5 stars (vs 4/5)
Slightly cheaper
Closer macro focus range (2cm vs 5cm)
Slightly faster shutter speed (1/1500 vs 1/1000)
Lighter by 1oz (30g)
Aside from a couple of those points, I'd say these cameras are practically the same thing. Personally, I love shooting long shutter photos, so that reason would be enough for me to get the canon (which I voted for). If you're into macro, then I would suggest the sony, because it lets you get closer to your subject.