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.
Prosophos 
I think that the equipment used means almost nothing. They look different, but neither camera or lens improve or ruin either image to any meaningful extent.
I agree completely. Well put. If anything it is the colours that are making me prefer one over the other so nothing about the image quality.
Mmmm…at 1000 ISO the Nikon performs better than M240 at 1600?! As in it holds itself together across the image better than the Leica.
But there may be something others see and once pointed out then I see it!
The black curtain looks much nicer on the M240 (Prefer this image)
Interesting reactions. I was going to observe the M240 has more noise but seems to have resolved more detail (though that could equally be a focus issue). I did a similar comparison with images from the Q and my D750 and while DXO will give you a different story I rate the images from the Q more highly from ISO 640 up to about ISO 6400. That said, a lot of that could be the lens on the Q, which is extremely good.
Simon, I agree with your observation that the M240-50/1.4 — as a “system” — has seemingly resolved more detail. It could very well be that the shot is better focused than the D810 one, but this finding holds true when comparing my other shots. I was surprised to see this.
Either way (focus issue or not), these images represent the output in actual use (my actual use anyway).
I think it is better lens (Leica) vs better sensor (Nikon). Both images look good to me.
Peter points out these are not scientific comparisons …the M240 seems to hold its own with the newer Nikon …holding out for a new CCD M, though the D5 is very tempting as the ultimate f mount action camera
Steve Huff predicts a new M coming soon, though he thinks it’ll be CMOS. What do u think Peter?
I would think that’s a safe bet.
I would have picked the M240 image as the nicer of the two to my eye. I own a D810 and shortly an M240. A Df would probably have done a better job than both although it was hamstrung by the inclusion of a less than epic AF sensor because Nikon knew they’d lose a lot of D4 sales if they gave it the good one!
Yes a D5 would be better but perhaps overkill for children’s parties…!
I had a Df and agree 100%, and would the Canon 6D as also bring better suited for low-light work. Lower resolution (16 mp for Df, 20 for 6D) has it benefits.
Interesting observations. So where are you at with the D810? Does the eye focus work and are you happy with the colors?
I’ve been fooling around with a D2x I got almost free as couple years ago and am really surprised how much I like the skin tones. I had also borrowed a friend’s D4 that I can purchase relatively cheap. So far, I really like the D2x skin tones. I realize it’s very old technology, but in good light I seem to get a lot of pictures I enjoy. I’m also seen more eyes in perfect focus.
John I really like the D810 (sensor, body, etc.), as you can probably surmise from the plethora of recent posts from this camera. And the tracking comes in very handy with my 300/4 lens — it is quite accurate. I can only imagine how good the D5 tracking must be.
As for the D2x, I always loved the output of that camera.
I think it would have been a much better comparison if you had used a 50mm Summicron or Summilux R lens with a Leitax F mounting flange.According to Leica, the R lenses, not the bodies , were some of the best they have ever made, espe ially those designed by Walter Mandler