The pixel difference isn't that huge. I don't mind. However, I actually prefer a little *bigger* camera! I think the S400 is perfect.
One other reason is the 3X zoom. The SD100 only has 2X. While I don't mind too much, a 3X would be nice.
Printing 4x6 prints, I can't tell a difference between the SD100 and the S400.
Though I have yet to take identical shots with both, I don't believe the image quality is too different. I don't have the most critical eye though. But I don't believe the difference isn't severe.
I'm perfectly happy with the SD100. My main gripe is that fingerprints appear on the body fairly easily.
If you're looking to not print beyond 5x7 or 8x10, save yourself the money and grab the SD100.