This is amazing that I see this post today. I am reassembling my gear after a year hiatus and just purchased both lenses in mint condition and now I have to make up my mind which one to keep and which one to sell. I read reviews and Sigma is getting hits on front focusing and being softer. My example of Sigma had been recalibrated at factory service for front focusing issue, by a previous owner, so I decided to do some testing
I ran the tests today, I set up a camera on a good Manfrotto tripod, picked the object, selected centered AF mode, and pointed camera to an object - my neighbors chimney

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Then I took shots at apertures 2.8, 4, 8 and 16 with Sigma, metering at center point and without exposure compensation, at 70mm and 200mm, using 2 sec delayed shutter, then I replaced the lens without moving the camera or tripod and shot with apertures of 4, 8 and 16 with Canon, again metering dead center.
I will post pictures tomorrow from work, Comcast modem took a crap and I am waiting for repairman to come over after tomorrow, I am using a work laptop with Verizon wireless card now, and to upload pictures using it is a *****.
So, I see that at 2.8 Sigma is somewhat soft, at 4.0 it is still a bit softer than Canon L - especially at 200mm. at 8 and 16 it's very hard to tell the exported jpegs appart, but looking at raw images in Lightroom, I can still see that Sigma is a tiny bit softer. On the other hand, Sigma is letting significantly more light in even at smaller apertures, with metering set at the dead center, Sigma pictures came a little more exposed that Canon.
Sigma is a great lens and you can beat the value for the money, I just need to decide how important f/2.8 for me vs. better AF of Canon.
At this point I am inclined to keep Canon lens and sell Sigma. pics will follow tomorrow.
that said, I had the same Sigma lens last year and took many great pictures with it at 200 mm, here are some examples: