I'm currently debating over either (Canon EF 70-200mm f/4L OR Sigma 70-200mm f2.8) OR (Canon EF 100-400mm f/4.5-5.6L IS OR Sigma 50-500mm f4-6.3) So, one of those. I can't decide if I want to extra reach or the glass speed. I hear good things about the sigma, but some say it's just not as consistent as Canon L stuff. Plus, the F4 can be found used for pretty damn cheap. But then there's the Bigma, oh, that range. But then the Canon 100-400 is appealing with the IS. Too many choices. Any opinions?