Ikelite Housing for S100
I have not used an Ikelite casing so far, but have three Canon casings (Ixus v3, G9 and G1X). Having seen and read about Ikelite casings, I understand that they are better when it comes to the use with external strobes and macro lenses. However, when looking at your user profile, I would suggest the Canon casing. The Canon housing is smaller (important when traveling and when in use) and will do very well for everything you want to achieve. Since both casings are made of acrylic, there is no significant difference in quality when it comes to your use - safe the 100 bucks and get the smaller casing.
Hope that helps.