So I find myself needing a fast zoom lens. I shot at a friends wedding last week with a borrowed 24-105 f/4L and always found myself needing to be a stop lower. The cake cutting was shot at 1600 ISO(even with flash it was necessary ) and has some noise to it. They hired another photographer so it's alright, but I was still upset. Anyway, in a few weeks, I'll be shooting another friend's wedding. I have the 28-135 that I'd like to sell, and that would help me afford the Canon lens, but is it worth it to eek out another wedding with the 24-105 f/4L and save up for the Canon lens, or save $600 and get the Sigma now and worry about the Canon later?