I have had the M240 for about 2 years now, and the A7rII since it recently came out (after an A7s).
It's hard to compare, really.
Leica M lenses on Sony A7* bodies work OK, some great, many not.. corners can be problematic even in the 35-50mm range.
So can't compare same lens on both fairly, and I don't really have comparable native lenses for each.
Sony Pros-
The A7* bodies are more ergonomic and lightweight for me, and my right hand has a ligament injury so for me this is a huge deal.
I also dabble in video a lot now, so A7rII has become my all-around camera when I want to go out and do a stills & video mix.
With the A7rII they seem to have finally figured out AF. I've shot MF for most of my photographic life, and mostly felt AF got in my way. With face & eye tracking, I'm now finding it convenient.
Popping into APS-C crop mode and still getting 18MP is cool.
Leica Pros-
I'd say pixel-level sharpness is probably higher. Also remember 42MP vs 24MP is not 75% more resolution, but 75% more pixels. Linear resolution is like 30-40% higher. Arguably 15% is needed to even be noticeable.
The files are just easier to process. Half the time I import into LR and am just adjusting on the margins with my Leica M 240 files. I have consistently found Sony files more work to post process. Whoever complained about M 240 color/white balance has not had to deal with A7rII files.
MF user experience cannot be beat. OVF has huge advantages for my shooting style, especially in 50mm. Watching outside frame lines to see subjects moving toward the frame, controlling my framing better, etc. Focus peaking and magnification and all the the other gizmos on EVF cameras are fine, but nothing like a Leica OVF.
The UI is just simple. First of all, the ISO/shutter/aperture controls are simple and predictable. Sony UI is a mess. Tabs of tabs of menus with sub menus. Stuff scattered all over the place, I have to remember the myriad locations of my commonly changed settings. Oh and the dials/layout on the Sony.. the wheel on the back is easily hit by your body while camera is on strap, and there's no easy lock/unlock.. so I had to just disable it entirely. Oh and for a movie focussed camera, they put the movie record button in the dumbest place possible, guaranteeing the first seconds/last seconds of every clip are a jittery shake-fest.
Leica has a nice methodical product management with history. Sony I feel like I am constantly being half-tempted by 95% great but often highly compromised products.. which are going to be superseded in weeks/months. The A7sII timing after the A7rII rollout was really bizarre.
Leica M mount has many great lenses at a variety of price points, speeds and weight from multiple makers. Sony FE lens line drives me kinda nuts. OK zooms, some good AF primes, some nice Zeiss Loxia, some nice Zeiss Batis.. all at staggered focal lengths & speeds so as not to compete.. but also to give less choice at each focal length. Yes I can adapt A-mount or Canon EF-mount, but that's suboptimal.
Leica M 240 battery is great. I bought a second one when I got the camera, but if you don't use live view, you really don't need it. I leave the camera to stay awake max time (5min?) before napping, and I can walk around with camera "on" taking shots for a full active shooting day and come home with 500 frames and 40% left on the battery. Sony I have 3 batteries. They actually ship the camera with 2 batteries, which says something. I've burned through 2 batteries in much lighter shooting. I swear in video mode you can basically watch the battery % tick down in real time as the record time ticks up..
Did I mention I love both cameras?
Oh people are doing that constantly.
If you do not crop the M240 with good lenses wins hands down I think.
But once you start really peeping all sorts of variables come into play. The Sony can only play with the other two at 25mm, 35mm, 55mm and 90mm where there are natives designed for the thickly covered sensor.
With those lenses the Sony will win any crop contest.
The D810 is the mature pro machine, and offers the only complete system of the three.
Now....what will Cosina come up with? 😉