This is a comparison I did tonight for my own purposes. I’ve decided to share it with you.
This reflects my “real life” use of these camera/lens combinations, but does not represent a scientific analysis.
I repeat, this is NOT scientific.
Here are two images from the kids’ school Lip Synch competition.
The first was taken in 2015 with the Leica M240 + Leica 50mm Summilux ASPH:
(f/2.8 | 1/250 | ISO 1600).

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The second was taken in 2016 with the Nikon D810 + Sigma 50mm ART (I was sitting a little further back):
(f/2.8 | 1/250 | ISO 1000).

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Now here are the central crops (looking at the individuals in sharpest focus from each image)…
Leica M240 + Leica 50mm Summilux ASPH (100%):

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Nikon D810 + Sigma 50mm ART (100%):

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The results above are consistent with other images I’ve examined.
The variables that differ between these two images are numerous. The only thing they have in common is that they represent real-life output from my use of these camera/lenses.
What do YOU think of the results?
—Peter.