Not exactly answering your question, but I've just done a comparison between my 50/1.4 AE and a borrowed Leica 50/2 Summicron M, both lenses in great condition. I've been a bit disappointed with the performance of my Planars at apertures wider than f/2.8, so I've been considering Leica M as a possible alternative for low-light work (I can't get on with the G series in terms of handling).
The tests shots were of a "real" subject (my back garden) using TMax 100 and careful focus bracketing was used. At the widest common aperture of the two lenses, f/2, they have a very different look. The Summicron kicks out a lot more contrast, and marginally more detail, but nothing to get up a real head of steam about. The bokeh difference is very surprising, and is where the Zeiss scores IMO. It almost seems to have a narrower plane of focus than the Leica, with objects in the far background being very fuzzily blurred out. The Leica lens renders these objects more recognisibly. Overall, the Zeiss lens has a very gentle character, low in contrast with some noticeable haloing around sharp features which in combination with the bokeh gives the image a dreamlike quality.
In conclusion, it's not the open-and-shut case I was half expecting for the Leica. I'm still mulling over the decision, but in the end it may be the lack of mirror slap which swings it for the M-series, rather than the optics.