After using Leica M cameras and lenses, when I bought a Canon 5DmkII, I expected that what I had heard - that Canon L lenses are on par with Leica M lenses - would be true. While it may be true for some people, I did not find it to be true for me. I think that while lens image quality may be quantifiable by lens charts and so forth,
apparent quality is subjective. In other words, you & I may look at the same image and both be equally convinced that image is positive proof of the high quality or lack thereof in a particular lens, regardless of what it does on lens charts.
I bought a 17-40mm f4.0L and the distortion was so "reckless" I returned it for another one that was better but still nowhere near what I expected from a top quality lens. So then I rented a camera bag's worth of lenses and did my own subjective tests using little or no science, the results of which
you can see here.
My own conclusions based on what I wanted from a lens in terms of image quality were that of the Canon L lenses I tried, the 24-105 f4.0L was actually a pretty good lens, but that the 35mm f1.4L was just a good lens. However, the Zeiss lenses - specifically the 35mm f2.0 T* ZE - were exactly what I expected from a high quality lens. The difference as I saw it was in micro-contrast that yielded at least the visual perception of greater detail and in color rendering which I saw as both truer and more thoroughly saturated in the Zeiss lenses than in the Canon lenses.
The advantage the Canon 35mm f1.4L has over the Zeiss 35mm f2.0 ZE is faster aperture and autofocus. But for my photography, those two things don't really matter, and what I saw in the Zeiss lens' images does.
Your mileage may vary, in other words. I suggest renting the lenses you are considering and seeing for yourself. Most pro-shops have rental, or you can get rental lenses on line from places like Lensrentals.com. Worth the rental price so you get what will work for you.