In this article, I'm looking at the out of focus rendering of the lenses, the bokeh. I also threw the Lumix X 12-35mm f/2.8 into the comparison.
The comparison was done by focusing on a close object, the box at about 50cm distance in the left foreground:
|Sigma 19mm @ f/2.8||Lumix G 20mm @ f/2.8||Lumix X 12-35mm @ f/2.8|
Here are some 100% crops from the centre:
And from the extreme right border:
The Lumix G 20mm f/1.7 features asymmetrical out-of-focus rendering outside of the centre of the frame: The further from the centre, the more elliptical the bokeh highlights become. You can see the same phenomenon in this study. This property is also shared with the Lumix-Leica 45mm f/2.8 1:1 macro lens.
The Lumix X 12-35mm f/2.8 also has some minor disturbance in the border: The out-of-focus highlights are rendered somewhat non-circular. When stopping down, this becomes less of a problem. However, this is a very minor issue.
Overall, I think the Sigma 19mm f/2.8 EX DN probably has the most pleasing out of focus rendering in this high contrast scene.