I currently have the 24-105 and 70-200 f/4 IS lens. I'm thinking about adding a 17-40 but don't know if it's worth it because I'll only be gaining an additional 7mm. Will the additional 7mm in width make a significant difference? I own both a 5d II and a 40D. If the difference is not that much then I'll probably go in a different direction and get the Sigma 50 1.4 for its low light and bokeh advantage. Unfortunately, I only have funds for just one more lens.