For those who shoot with Canon... I am looking at the Sigma 100-300 mm f/4 vs. the Canon 100-400 mm f/4.5-5.6L (....I know, the f/2.8 is better but I can't afford that right now). I've heard the Sigma hunts a bit when focusing and I am shooting outdoor sports, so that would be an issue. Question #2: how much of a difference is there between f/4 and 4.5?....so how much do you gain with the Sigma as opposed to the Canon? Any feedback is appreciated. Thanks!