This tired old debate has just resurfaced on a post in a dpireview.com forum. I have shot with Leica lenses in the past, owned several Zeiss optics on a Contax 35mm (as well as shooting Hasselblad), and have owned and used Nikon lenses for over 40 years.
All of them … Leica, Zeiss and Nikon have produced perfectly satisfactory results. None of them have had me screaming “look at that photograph … I must have all Leica, or all Zeiss or all Nikon lenses.”
I am sure that exhaustive, and scientific testing could see differences between them in terms of sharpness, contrast, bokeh (but who really gives a stuff about this? I shoot for the subject not whether or not the out of focus background looks prettier in one shot than another.The cult of bokeh is an annoying irrelevancy) colour rendition, maximum speed and so on.
In the final analysis though, it is the image that counts. Millions of photographs have been taken, and millions reproduced from all three and I have yet to hear “Wow? I bet that was shot with a Zeiss! Or a Leica! Or a Nikon!” Most people couldn’t care less. And that’s the way it should be.
Buying one over the other won’t make you a better photographer. Lenses, like cameras, are only tools. And using that as an analogy, do you ever wonder if that screw in the door hinge was inserted by a Makita, or a Bosch, power screwdriver? Of course not, so why fret over the make of lens. If you truly want a Zeiss, or a Leica, by all means buy them … but I wouldn’t sell a Nikon to do so. Any improvement would hardly be worth the money, and don’t expect an improvement in your photography.