I went through the same decision process a few years ago. At that time the Sigma and Canon lenses both received excellent reviews and user comments. These lenses are frequently discussed and debated on dpreview.com. I suggest you check the Canon Lens forum on that site as there are a few folks that either have or had both. If I had to summarize the comments I continue to read (dangerous thing to do), it seems both are pretty comperable in overall image quality.
I decided to buy the Canon version with IS, more than anything else the IS made the decision for me. I expected to do a lot of handheld photography with somewhat slow shutter speeds. The IS adds a lot of cost to the lens, the question is if this feature will be important to you. If you’re shooting with relatively fast shutter speeds, sports, or on a tripod/monopod then the IS might not be important. In my situation I’m glad I decided for the IS and have been very pleased with the lens overall. I've never used the Sigma.
A lot of the shots in these galleries were done with the Canon 70-200 f/2.8L IS, some with 1.4 and 2.0 teleconverters, most are handheld and a few are with a monopod.
http://www.pbase.com/dougj/birds_birds_birds
http://www.pbase.com/dougj/fauna_mostly_asia
I hope this helps.