I am seriously looking into buying a Canon 17-55 EF-S F/2.8 Lens for my 50D. I noticed that Both Sigma and Tamron make a similar lens. Does anyone have any suggestions as to which one they would buy? Also I noticed last night that Sigma (I think) makes a 17-70 (or somewhere around there) F/2.8 lens. Any thoughts? The prices are pretty much all over the spectrum so I am really confused here.