If I had the money I would buy the Canon 70-200mm MKII, and be done with it. But for my use(non-professional), I can't justify spending that much. I haven't had a chance to try any of the Sigma 70-200mm versions, so I can only go by reviews online. I have tried other Sigma lenses (Bigma, 70-300 apo macro, 10-20mm) and have been happy with them. I never even considered the Tamron 70-200mm just based on the focus motor. But, after reading about it's optical performance I took the plunge. I love the quality of the images, and can manage with the focus. I thought about ordering the Sigma and the Tamron at the same time to compare and keep the one that felt best..but I didn't. I think I would buy the Tamron again. Still, I would have loved to at least test the Sigma. If Tamron releases a version with their USD motor, I would buy it (as long as IQ doesn't take a major hit). I also thought about going for the new Tamron 70-300mm...but the constant 2.8 was what I was really after.