Sorry for the late reply guys.
I don't check the forum as often as I'd like to lol.
I actually own both lenses. It is hard to compare them since they are quite different in rendering.
But i must say for the past 2 years my 40mm Cron has been my most used lens. Especially when it comes to B&W film, the lens shines with its sharpness and character, and i find the bokeh wide open to be classic and beautiful.
However, the more I've used the lens these past 2 years, the more i see some of its faults, but mostly when trying to use it on digital. For one thing, being a vintage lens, it has the classic vintage Leica character of having a certain glow wide open. But i only see this when there is strong, bright lighting. Several times I've taken the lens on family trips to the park or to the amusement park, and in bright midday sunlight, the highlights tend to glow and sometimes distract from the overall image. But like i mentioned, i only see this glow in digital. In film, the lens is near flawless to me.
But after using the lens for so long on digital, i finally decided to try a more modern lens for those difficult situations when i know modern glass and coatings will help. Thus the Zeiss Biogon 35/2 is now practically glued to my M240. It gives the perfect aperture to fit my girls in the shots, and performs well in all types of lighting, even the harsh ones. So editing digital images has become much easier lately.
I guess in short it all depends on what you mostly plan on using the lens for. Like others have mentioned, the 40mm will give you classic character, and the Zeiss will give you modern performance and creamy controlled bokeh.
I do own other 35mm lenses that deserve honorable mentions though!
The Canon 35mm F1.5 is an amazing lens. It performs excellent wide open with beautiful classic bokeh, and a sharp center. However it also has the vintage glow, more so than the 40mm. And also wide open, the lens has strong vignetting at the corners, but in my case it might be the filter that is too thick?
Another wonderful lens is the Summaron 35/2.8.
It is a stop slower than the classic Cron V1, but it perform beautiful like a modern lens would. The colors it produces even in digital are excellent, and the sharpness is great as well. Bokeh might be lacking a little, but it is still great for most situations.
Well that was just my 2 cents on the matter. But like Raid mentioned above, there are sooooo many options when it comes to M mount 35mm lenses. And all of them have their strong points. Enjoy the hunt!