Here is a link to specifications on the five:

These cameras are very much alike.

The Nikon S5 and S6 are identical except that the S6 has a 3" LCD and it has built-in WiFi.

The Canon SD600 and SD630 are identical except that the SD630 has a 3" LCD (but no viewfinder). The SD600 has a 2.5" LCD and a viewfinder.

The most note-able differences are not shown in the specifications.
Shutter Lag:
Canon SD600 - 0.1 second
Canon SD600 - 0.2 second
Nikon S5 & S6 - 0.6 second
Nikon S9 - shutter lag unknown (no reviews on this camera)

While looking at sample photos that are in Steves-Digicam reviews of the Nikon S5 and S6, I see what appears to be a manufacturing/quality-control problem.

The very right edge of the S5 photos are out of focus.
The very left edge of the S6 photos are out of focus.

If I bought either of those cameras, I would want the option of returning them for a refund if I found the focus problem.

The S9 is an unknown since it has not been reviewed. But it does have a different lens. It may not have the same type of problem as the S5 and S6.

The Canon SD600 and SD630 photos are a little softer than the Nikon cameras, but there is no out of focus problem on the Canon camera.

The Canon cameras seem to be the safer choice.