Ok, so I'm sitting here looking at both lenses, and I can nab the f4 almost 200$ less (or more) than the equivelant sigma. I've shot with the Sigma version, and having the slightly faster lens certainly would be a nice thing. I'm seriously considering, however, that only spending 500$ on the used L , then nabbing a nice wide angle to compliment the bottom range, would not be a bad idea. I'm building my lens kit from scratch this year, And I know that 70-200mm is something I want. I very dearly want to break into some proffessional work(and have the aid of a professor at A&M to help ), so advice would be appreciated. My primary goal with this lens is to have a general purpose "long lens", wiht the option to throw a teleconverter out in good sunlight to shoot with.