There are several differences.

Here is a look at the specifications:

http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_sd400%2Ccasio_exz55&show=all

As to the prices you listed.....I would say that you have found the Casio Z55 on sale. It's price is usually close to $300.

...

extra warranty:
50% of the people say yes.
50% of the people say no.

Shot to Shot time...
SD400 is 1.5 seconds
Z55 is 2 seconds

...


Continuous Shooting or Burst mode.
SD400 2.2 shots per second in continuous.
Z55 - does not have continuous or burst mode.

...


I would recommend both.

....

I would ask those people that say "wait"....

"Do you promise to make up the difference if it is not cheaper after Christmas."

...
..
.