I've owned both and while the Canon does have a slight edge over the Siggy's sharpness, I would pick the Siggy anyday due to the cost savings. If money was no issue, of course, I'd pick the Canon... but given the ~$400-500 difference (used prices), I'd definitely take the Siggy.
Sigma's HSM is very comparable to Canon's USM, the Sigma 70-200/2.8 is no slouch and IMO is very capable of shooting sports.
You could always do what I did... buy the Sigma used, and continue savings... Once you get enough, sell the Sigma and buy the Canon. Good glass prices stay pretty constant and as long as you pay close to market rate on the Sigma, you shouldn't lose any money.
Thanks for the advice, sounds reasonable. Now to find a used siggy.