I started reading this thread with interest and wanted to chime in regarding the "reasons to switch" to a mirrorless A7R. I can only speak for the A7R since it was the choice I made.
First off, I am a pro shooter. Prior to the A7R purchase, I owned (and still own) a full Canon system with a plethora of L lenses centered around a 1Dx. I also own a Phase One medium format system with a Leaf Credo 60 digital back.
I had a Sony NEX7 as my carry around camera and moved to an A7R instead.
Why?
Well, several reasons...
I wanted a full frame camera instead of APS-C.
I liked the addition of a battery grip.
I wanted a real menu system.
I wanted 36MP.
And the REALLY BIG ONE..
After looking into the entry cost of a tech cam to shoot interior architecture using my high resolution digital back (60MP) for billboards at a cost of about $10,000 all in, I realized I could get a reasonably high resolution using my stellar TSE 17 and TSE 24 II lenses on a $2000 A7R with a Metabones adapter.
Using these same TSE lenses on my 1Dx yielded only HALF the resolution.
So, I have an excellent, lightweight carry around camera that doubles as a tech cam with my TSE's. Win win.
Does the A7R perform as well as the 1Dx? Not by a long shot in terms of speed and accuracy. Different tools for different purposes. Is the file size as large at the Credo 60? Nope. Different tools for different purposes.
But, stitch three TSE shifted A7R interior architecture shots together and the Canon can't touch it. Right Guy?
And one final HUGE thing...
Using the built in WiFi in the A7R and the free iPad app, I can wirelessly connect and tap on my iPad to select any point on the screen to focus. It grabs focus and beeps and I take the shot. Wirelessly, flawlessly, every time. It is like a built in "Cam Ranger" (look it up if you are unfamiliar).
For architectural photography, I can walk around with my iPad and a flash and pop light where I need it holding an iPad and a flash and immediately see the results on the iPad. I can change aperture, shutter speed, ISO and tap focus points on the fly using my iPad.
Priceless!
This ability is not available with Canon or Nikon unless you buy a third party, bulky, Cam Ranger.
Those are my reasons for the investment.