The question is somewhat ill conceived. I have both systems, and although obviously they overlap, they are not the same.
Zeiss Ikon is foremost a low light camera for me. It is the best 35mm FL body there is around, and paired with the Nokton 35/1.2 is simply unbeatable for low light film photography. Zeiss Ikon is also a better camera for the street than any film Leica, for the 1/2000th sec shutter, which gives you extra flexibility with exposure. On the flip side, Leica has the 0.85x and 0.58x magnifications, that are better suited for 50 and 28mm - in fact I have 2 M7's (0.85x and 0.58x) for these focal lengths.
As to the Zeiss lenses, with the exception of C Sonnar, they are very high contrast, and unless you like that look in B&W, I feel that they excel in colour photography. You can mitigate the problem somewhat by exposing richly and pulling your film, but this has its separate consequences.
The final point is, the reliability/durability/fondling factor. Although my ZI works flawlessly, somehow Leica bodies give you a psychological feeling of being a more reliable tool, and this is not necessarily correlated to the effective maintenance problems with the cameras.
I would not want to part with my ZI, but I would not want either to have it as the only M mount body.