I have the sigma 12-24 and canon 24-70 which cover the wide angle range on my 5DII. However, I find that I need to swop these lenses around quite frequently. The 17-40 would fill the gap but don't know if I can justify the outlay when I already have the range covered. One thing that may tip the balance is IQ. Does anyone have experience with both the 12-24 and 17-40? I'm interested to know if the 17-40 is significantly better in IQ.