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Good advice above, IMO.
For macro work, the IS will generally not make enough of a difference. You will still want to work mostly on a tripod and/or with plenty of light.
The Canon 100 Macro 2.8 (non-IS) is a great lens. I have had two. Very sharp, fine portrait lens, and I have even done some landscape shots with it. And the price is right. Check BS. Probably cost you $400-450 used. I dont know the Sigma but have heard good things.
What else are you planning on shooting? Following this advice, you will have about $600 left over!