As Pookie pointed out, I sold my 16-35 f/4 IS not long after purchasing the 11-24 f/4. I am not a big filter user, I use a tripod 95% of the time (with this kind of lens) and love shooting very wide, so it works well for me and I don't think the 16-35 would have seen much use, so I thought it would be better to sell it to someone who would use it. I also have the EF-M 11-22 IS for my EOS M, so to me that was a lot of overlap.
Comparing the two, here's what I thought:
-Build quality is slightly higher with the 11-24, but I don't find it to be much over the 16-35, which is excellent
-Image quality is even or very close (100% pixel peeping required) between the two. Color, contrast, and sharpness are all top notch on both lenses
-CA is a bit lower on the 11-24, but both a corrected quite well
-Distortion, especially at the edges is lower on the 11-24
-Vignetting is more even on the 11-24 - the edges are quite dark at 16mm of the 16-35
-11 to 15mm is not for everyone. It's quite difficult to find a suitable composition, and even then, the camera needs to be carefully leveled and/or angled to get good results
-Corner stretching/projection distortion is obvious, but can be left alone or (once a profile is available) corrected in DxO. It will result is a ration other than 3:2, but it's very usable.
I see the 11-24 f/4 as a true specialty lens, whereas the 16-35 f/4 IS is much more of a general purpose wide angle lens. Like most specialty lenses, it is expensive and there are trade offs in terms of size, weight, zoom range, filter use, etc. It's not a practical lens and you'll find yourself changing lenses a lot if you're walking around or doing general shooting. 24mm is just too wide for a whole lot of subjects, whereas 35mm is just right for many subjects. Plus, there's no IS, it's heavy, and it gets lots of attention (which made me a little nervous when I was surrounded by several homeless people a few weeks ago at night).
For wedding and event shooters, the 16-35 f/4 IS of f/2.8 II make way more sense. For travel shooters, the f/4 IS along with one or more of the 24-70 f/4 IS, 70-200 f/4 IS, 70-300 f/4-5.6 IS lenses are a killer combo, and in many cases the 16-35 f/4 IS could be a single lens solution, or maybe it and a 50 or 85mm prime for portraits and low light. The M + 11-22 IS works quite well, too
For landscapes, the filter usage is a huge deal, and honestly, not a lot of natural landscapes are well suited to being shot at 11mm. It makes the background SO small (see
Ian Plant's example from Patagonia to see what I mean), but will certainly find uses where the foreground and creating vistas from shallow compositions is needed. HDR and other digital tools are going to be more important, of course, and it's huge and heavy, so not ideal for hiking or stashing in a small bag. It's also nowhere near as weatherproof as the 16-35 f/4 IS + filter.
For architectural/real estate shooters, I have the TS-E 17 f/4 and the TS-E 24 f/3.5 II as well, but see them as very different lenses in terms of capabilities. The T/S have obvious advantages, but aren't weather sealed, aren't zooms, and require stitching to get a wider FOV. That's usually doable, but a lot of my work is done under short timelines (owner is coming back soon, or realtor is waiting on me) so being able to zoom from ultrawide to a standard wide FL is quite handy. I never found the 24-35 range to be very useful for interiors, so that's not a big loss.
So ultimately, for me, I'm willing to trade the major conveniences of the 16-35 f/4 IS for the unique perspectives of the 11-24 f/4. For most people, however, the 16-35 f/4 IS makes way more sense and is one of Canon's best lenses in every way possible.
P.S. I would caution anyone comparing IQ to do it between f/8 and f/16. There are few reasons to shoot wide open with these lenses and the IQ, even with the Sigma, is better compared stopped down where these lenses will most likely be used (sports and event shooters aside...but then again, they need f/2.8
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