Well, you get what you pay for, so I'm not comparing lenses that are going to cost 4-15X more. A fair comparision should be with equivalent lenses from 3rd party with similar prices. The Sigma 70-300mm APO DG Macro in my opinion has a slightly better sharpness and contrast, but the difference is hardly noticeable. What I hate about the Sigma is that the focusing speed is so slow, and it really hunt in low light. I have tested both Canon and Sigma lense under similar conditions after my bro-in-law complained about his Sigma's slow speed. The Sigma do have an advantage of having a closer minimum focusing distance, thus the designation of macro. Maybe my bro-in-law's Sigma could be a bad copy.
