. . . is the Canon 17-55 IS than my Sigma 17-70? I'm looking to upgrade either this combo or my 70-200 f/4 for the 2.8 IS. I use the Sigma much more than my 70-200 but I'm more satisfied with the Sigma than I am the f/4. Anyone help me out here about which to upgrade first? Anything I'm not taking into consideration? Thanks