It depends on what your priorities are. If you just want great automatic pics without doing any fussing, I'd say the Canons are a bit better. If you want great quality movies as well, the Casios would be a good way to go. I like my z750 and z850 because you have so much control over the pictures. Battery life is great, burst modes and startup times are very nice, and video quality is great. Also, the Best Shot modes are quite useful when you learn how to use them. Since I don't have experience with the s500 and s600 I can't say, but 2 of my friends have them and love them. All of those cameras are great, but I'd say:
Best for Simple Automatic-Canon
Best all around-Casio