It's time to buy a ultra-wide angle lens for my full-frame 6D. The 2 contenders that I am considering are the Canon 16-35mm f/4L & the 17-40mm f/4L. The obvious differences between the 2 lens are IS (on the 16-35) & the price. Neither of these factors are game changers for me. My primary concern is IQ on the 6D. With that in mind, I would appreciate your thoughts & recommendations. Thank you so much!