I own both. I bought the 17-40 as one of my first lenses for 5D, and used carefully it gave me very good results. Contrary to earlier suggestions, I find it to be very good at controlling flare, it is sharp in the center, and stopped down focused close it gives corners that sharpen OK in post, but only stopped down. More distant details in the corners are a bleary mess. If it is grass close in the foreground you might be OK, if it is clouds you might be OK, if it is tree branches farther away, forget about it. I pretty much use the lens at f/11 period. I took many pictures that I like a lot.
I make large prints and ultimately I became dissatisfied with the level of detail the 17-40 was able to give, so I bought the Zeiss 21, and it does what others say it does. For landscape I still use it stopped down, but it is sharp without excuses.
It is also f/2.8 and can isolate foreground from background in a way that I was frankly never tempted to try with the f/4 zoom.
The 17-40 is light weight and versatile and it can do 17mm, the Zeiss 21 is big, heavy and 21mm only. If I were going to Europe, I would probably tale the 17-40 and leave the 21 home. To me, the ZE 21 cannot replace the 17-40, but if 21mm is what you want, the Zeiss is for sure better.




