Between the two, the Sigma 100-300 f/4 is a tad shaper, but the 70-200 f/4 has been a favorite of mine (till I got a 120-300), in my opinion, for the price of the Sigma 100-300 f/4, you can pick up a used 70-200 f/2.8 canon L. (and since you like sports/cars, the faster f-stop would be used and welcomed).
(the Sigma 100-300 f/4 and Canon 70-200 f/4 both are excellent lenses, i have yet to hear any Canon users complain about either)