I purchased the Sigma 70-200 OS and returned it, then bought the Canon 70-200 IS II instead.
However, I did so because the 70-200 OS was so freaking amazingly awesomely good that I fell in love with 70-200 lenses and just had to have the best available. Comparatively there is relatively little difference though. The Sigma AF is just as fast and accurate as the Canon. The IQ, especially on APS-C, is very close as well. The Canon is better, but it is really not $1100 better (I got mine for $900 more). Only if you have professional reasons for needing the absolute best, or truly money to burn, can I recommend the Canon over the Sigma..... And even then, bear in mind that for the last 10 years the best Canon lens available at 70-200 was basically no better than the Sigma, and that lens was a top-seller and incredibly popular. If it was good enough for everything it was used for, then the Sigma is good enough too.
If I were you I would seriously consider getting the OS and using the difference to get or save for another lens to go with it.