Which Lens?

I am a semi-proffessional photographer and in preperation of shooting my first wedding this summer I am purchasing some new lenses. I'm a bit bogged down by all of the choices but have narrowed my long-distance lens down to 3 choices: the Sigma 70-200mm f/2.8 constant ap throughout zoom with the optical stabilization system, the Canon 70-200mm f/2.8 constant ap without IS and the same with the IS. Does anyone have any experience shooting between the Sigma with OS and the Canon without IS? Is there a big difference? I'm sure the best would be the Canon with IS but the Sigma is about $500 cheaper, would the Sigma optical stabilization be comparable to the Canon image stabilization? Are there any other lenses that I haven't found in my shopping that would be better in that range?