Well I sold a 40D, 17-55 f/2.8 IS, and Sigma 10-20, and will be replacing it with the 5D mkII, 24-105 f/4L IS, and [insert decision here]. I'm leaning for the Canon 17-40 f/4L for the following reasons Familiar range (relatively comparable to the 10-20 on a crop) Range is better for an everyday shooter on a crop body The coveted red line. I've been moving to mostly Canon grub for awhile, and like it. Price ($650) But, the Sigma 12-24 sounds pretty intense, being so much wider and working on a full frame. People seem happy with it. It costs $780. Anyway, I'm leaning towards the 17-40. What would you do?