I would rate the 21mm above the 16-35 f4L IS at its focal length and you have the 2.8 over 4.0 advantage. There is also something special with the Zeiss glass that makes them stand out. Color and contrast are two.
But apart from that, the IQ from the 16-35 is so good that it even seems a bit pointless to hang on to the 21mm. And when you add the weather sealing advantage of the L-lens, it is even clearer. Since I got the 16-35 I have hardly used the 21mm at all. If I had to choose only one of the two, I´d go for the 16-35.
I don´t have any good images to serve as comparisons though.
Don't worry about comparison images, i trust your opinion as an informed user.
Weather sealing on the Canon is a nice plus, but not a game breaker in my typical use case scenarios. Same goes for the IS. I won't be needing the fast aperture either, since i mostly shoot long exposures. What appeals me about the Zeiss, other than the stunning image quality, is that it's built like a tank, and the fast aperture would allow me to get started in night photography: star trails are cool, but being able to take pictures of "still" stars would be great too. Not to mention that the infinty focus hard stop is really a life saver when composing shots in the dark. Now, if only it wasn't so freaking expensive... For the price of the Zeiss, i could buy the Canon and a Rokinon/Samyang prime for star fields, but i'm adding complexity, increased lens switches and weight. Or, i could get the Canon 24/1.4, use it stopped down at f/2.8 for star fields, wide open for low light/street photography, and stopped down for landscapes.... Ah, choices, choices.....