I went through a similar process to what you are doing now. I had the Canon 60mm macro and wanted to upgrade. I chose to go to the Canon 100mm macro IS USM and whist I agree with Markulous that the IS only gives you 2 stops, I do find the stabilisation useful when using my camera handheld and the lens as a 100mm prime. It is a beautiful lens to use, once you get used to it.
I just thought that if I bought the non-IS version of the Canon 100mm I'd end up kicking myself. BTW I'm not knocking the Sigma, as I didn't look at it in the end.
There's a review of the Canon lens here on PhotoRadar
http://www.photoradar.com/reviews/pr...-is-usm-review
I hope this helps.
Regards
Nick