The S100 has a larger sensor and should provide better image quality, especially at high ISO. It also allows you to shoot in RAW mode for best image quality. The Elph 500 HS is smaller. It really depends on what's important to you.
I found a Canon PowerShot 500 HS for $169 with free shipping, so which one?
|Waffles with fruits by Coolinarka|
from Food photography (desserts)
|Vestrahorn Frozen Reflection by Will B Milner|
from Ice cold