Take it from me. I've been shooting Canon DSLRs since 2006 and have owned two 16-35 2.8 IIs, and probably half a dozen 17-40Ls over time shot on full frame 5D mark I and Mark II bodies as well as Rebels and a 40D.
The 17-40L and 16-35LII are optically Canon's best ultra-wide zooms. They both perform equally as far as sharpness goes and both look identical stopped down. (the photo on the front page of my site was shot with the 17-40 and 5D2 and even at 1900 px wide, that image is super sharp corner to corner. Shot at F/22 too!
The 16-35 Mark II shines obviously in low light and wide open is SUPER SUPER sharp in the center (excellent for casual/fun portraits) it's just a JOY to use in ALL situations whereas the 17-40L is a joy to use in SOME situations. the 17-40L is softish wide open especially in the corners. the 16-35LII shows excellent center sharpness wide open and good in the corners.
Is it worth the extra 7-800 bucks? YES YES YES. I am so happy with my 16-35L II and it's going to stay with me likely forever.
Thanks a lot for this mini field review. Although my signature still says something else, reading these things I can imagine to go for the 16-35 instead of waiting on a phantom. As I do low light photography wide open, your experience is crucial for me. So I better burn an additional US $ 280.00 for a 10 stop ND filter instead of a 1000 more for a highly priced WA-zoom that doesn't even exist
Your insightful post is highly appreciated. Cheers, Pedro.