(Not a scientific comparison between images taken at two different times under different conditions — but this reflects “actual use” conditions for me.)
(See related: Nikon D810 vs. Leica M240 in “real life” use.)
I photographed my kids’ school Fun Fair yesterday.
Here is an image from the event, taken with the D810 + Sigma 35mm 1.4 Art @ f/4:
Here is an image from last year (2015), taken with the Leica M9 + Leica 35mm Summilux ASPH FLE @ f/4:
… And here are the magnified views.
First, the D810 + Sigma 35mm f/1.4 Art @ f/4 (magnified):
Next, the Leica M9 + Leica 35mm Summilux ASPH FLE @ f/4 (magnified):
The D810 + Sigma 35 Art made it easier to photograph the event, the M9 + Leica 35 FLE produced more pleasing image quality (in “actual use” conditions).
In all fairness to the D810 + Sigma 35 Art, when the conditions are right and the focus is nailed, this combination leaves very little to be desired: Nikon D810 + Sigma 35 Art in optimal conditions.