I'm looking at both the Sigma 50-500mm f/4-6.3 and the Canon EF 100-400mm f4.5-5.6L IS. I like that the Sigma has a wider zoom rage and is a little cheaper, but is the lens just as good as the Canon? I'd rather pay the extra $600 for the Canon lens if it has a much better image quality then buy the Sigma and then wish I had the Canon. I'd use the lens primarily for wildlife photography.