Here is a comparison of the two cameras specifications:
Notice that there is a link directly below the red stars. Click this link to see owner's opionions about each camera. Read them all.
I looked at photos of the same subject, from both cameras and the photo quality and sharpness is just about equal on these two camera.
I'm trying to pick between the Casio EX-S500 and the Canon SD400. I've seen the specs for both on dpreviews.com, but does anyone have any positive or negative experiences/opinions about either?