Both cameras take excellent photos.

I would rate the picture quality a tie.

But I wonder why you chose the 7900 instead of the 5900. They are identical cameras except for the megapixel count. The difference in price is about $90.

If you are planning to print large photos (larger than 8 x 10) the 7900 would be the better choice.
If not the 5900 would be the better choice.

When comparing either of the Nikon cameras to the Casio Z57, the Nikons look better to me.

While the Casio has the very large LCD,
it has the weakest flash,
and has no continuous drive (burst mode) which is essential for sports/action shots.

Here is a comparison of specifications (side-by-side):

http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=casio_exz57%2Cnikon_cp7900%2Cnikon_cp5900&show=all

........................

As far as extended warranty.......
It is the same as health insurance.
If you get sick it is good.
If you don't get sick you spent money unnecessarily.
Do what seems best to you.
...
..
.