Wondering on opinions on these 2 lenses. I currently have the Tamron f2.8 70-200, and I am looking to replace it. I generally shoot a lot of motorcross and outdoor photography. I currently have the Canon 60D, but am looking to replace it w/ full frame depending on what the 5D mkIII brings. I know the Canon 70-200 mkII is an awesome lense w/ very few problems, but when I do switch to full frame I will be loosing the equivalent focal length of 105-320 (if my maths correct). So would the Sigma 120-300 maybe be a better bet? The cost of the lenses is pretty close ATM w/ local retailer selling the Canon for $2529 and the Sigma for $2999. Just curious on other's thoughts.