I'd like to know if anybody who has used these two lenses can tell me which one they think is superior. I've searched photo.net and Google, and have found some reviews which claim that the Tamron is better, and other reviews which claim that the Sigma is better. Basically I'd like to know which one is sharper and has better contrast. I've also heard that both of these lenses are comparable to, or perhaps even better than, comparable offerings from Canon. I realize that camera-brand lenses are usually superior to lenses made by companies such as Tamron and Sigma, but I suppose anything is possible. Lastly, can anyone comment on the Canon 55-200 4.5-5.6 lens? I haven't been able to find much info on that one. For instance, I'd like to know how sharp it is at 200mm compared to the above lenses at 200mm. Thanks!