What do you prefer? Obviously a do-all is more convenient, but is it worth it? Is swapping lenses all the time worth it? I've got the Nikon 18-200 VRII right now and the versatility is great, but I'm considering covering that focal range with a few lenses instead. Thinking about these: Sigma 10-20 f/4-5.6 Sigma 24-70 f/2.8 Sigma 70-200 f/2.8 What are your thoughts?