I'm looking for a wide/standard zoom for my 7dmk2 and narrowed down my desired focal length to 16-35mm.
From what I have read so far the 4.0 IS version is newer and sharper near the edges. I have also read the build quality if the 2.8 version is much better (metal filter thread instead of plastic, stronger outer casing, etc.).
I am split between the amount of indoor/outdoor shots I take. Having IS or not is not a deal breaker for me as I can always up the ISO if needed.
The main intended use for this lens will be travel, occasional indoor sports, occasional portraits, some landscape; hand held at all times.
While there is close to a $500 difference between the two, if the 2.8 is truly that more spectacular then I will wait until I can budget for it. I tend to keep my lenses for a very long time, rarely selling any of them. Also, while I am happy wirh my 7dmk2 for now I won't discard the possibility of someday going full frame.
ps. I want to stick with Canon as I have never been a fan of Sigma due to previous negative experiences with Sigma lenses I have owned.