I used to have the Sigma 70-200 and now have the Canon 70-200 f4 IS. The focusing on the Sigma is lightning fast; I couldn't really tell the difference between it and my Canon. If you need f2.8 and your only hesitation with the Sigma is the focusing speed, then get the Sigma. It's a fine lens. I sold mine because (a) I found it too big and heavy, and tended to leave it at home more often than not, and (b) the lack of IS was a problem for me. If you haven't actually handled any of these lenses, I encourage you to do so before buying. There's a very big difference in size and weight between the Canon f4 and the Sigma f2.8. This doesn't bother some people, but it did bother me.
I did try the Tamron before buying my Sigma, and noticed a difference in focusing speed; the Tamron was slower. Given that I wanted the lens mostly for sports, I chose the Sigma.