Hello,
I had the same choice 2 weeks ago and indeed, the Zeiss is the Grail. Perfect at all apertures for its contrast and definition, smooth bokeh. For the resolution, the Summilux 35 asph FLE is not able to do the same results at 1.4 for the Zeiss until f/4 for the Leica ! The Zeiss is probably, theoretically, the best 35mm ever.
But… I eventually bought the summilux 35 asph FLE. So Why ? I am not a Leica worshiper and the price is an issue for me. It is because of the global rendition, wich I find better. Sipmly to take more "human" pictures. Especially some shots of jonmanjiro (who prefer the Zeiss, if I understand right) on Flickr in Japanese markets or restaurant where in my opinion, the Summilux shows an unequal transparency and contrast. Moreover, I can’t explain exactly why, I found the smoother bokeh of the Zeiss not natural.
I made few years ago a comparison between the latest Summicron 50 and the famous C-sonnar 50/1.5 and both were nice, but the Summicron shows clearly more detail in shadows and better color rendition, especially in low and artificial light. It’s like some wavelength have been absorbed by the Zeiss lenses. Sorry, I don’t have pictures since my hard disk died…
So maybe it is because of less detail in shadows, and associated with it the out-of-focus rendition, I preferred the Summilux. And about the summicron ? Well the Summilux is clearly better : For the resolution, the two shows to my eyes the same results. But for the contrast, the Summilux is better : it shows deeper shadows with more details at f/2. And at 1.4 too ! But with a lot of vignetting. The bokeh is approximately the same. The better in it aspect is to me the previous Summilux Asph.
I don’t know well the Voitländer lenses, I have a 90 apo-lanthar and an Heliar on a Bessa iii. These are bargains and top-class for the resolution but I don’t use them so much : to me the Leica or Zeiss shows better detail in shadows, color rendition, contrast. The lens I buy are for decades so psychologically speaking, I don’t want to use ones that don’t show the best for the people I love.