I would suggest reading the reviews for all three on this site. I haven't used the Sigma or Tamron, but I can vouch for the Canon f/2.8 and f/4 as top notch in every regard.


The review here indicates that the Tamron autofocus is lacking. The Sigma has HSM, but all three copies Bryan tried had front-focusing issues. If you have a body with microadjustment, you can correct for that.


f/2.8 is important for sports, so I think you're on the right track trying to get it if you can.