the canon 100 macro doesn't have IS, which makes a huge difference when doing handheld macro photography. The sigma has it--so much easier to use. If you're going to do all of your shooting from a tripod, then it won't matter. I'd also take the build of the sigma over the canon, but either way you're not going to have weather-sealing.
Last edited by AngryNeeson52; 01-26-2014 at 10:51 PM..