I have owned both of these lenses and wanted to share some insight for those who are interested. The 19 was adapted by fotodiox and both lenses were used on a 5DII.
The 21 is much sharper corner to corner wide open than the 19. The 19 is wider and has a deeper perspective shift. The 19 really shines at f/8 and f/11 although I have some wide open shots that show a pleasing bokeh considering. The 19 is smaller than the 21 and has a built in filter set. The 21 takes 82mm filters. The 21 has mustache distortion which is a 2nd order distortion while the 19 has barrel distortion which can be easily corrected in post. The color fidelity and IQ of both lenses is excellent. In terms of perspective I prefer the 19. The 19 has a rear lens guard that protects the rear lens element. This must be modified to work on EOS cameras. I recommend SK Grimes for professional lens work. $200 or less. The build quality of the 19 is excellent and it's a classic manual lens. I had a ROM version. The 21 is far more accessible for the EOS user.
I will post a few photographs as I find them.
Leica 19
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Zeiss 21
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