I have a question: I am about to buy a Sigma 28-70mm lens with aperture varying between f2.8 and f4.0 between the short and the long end of the focal length. The cost is an amazingly cheap $139. I was getting ready to buy it and saw another lens by Sigma, just like it, except it has a constant aperture of f2.8, but it's an additional $200. Should I wait and get a faster lens? Is the 1-stop advantage really worth it? Thanks!