I just viewed Fred Miranda's site and he has a comparison between the Sony a7R and the Canon 5 EOS Mark III. http://www.fredmiranda.com/A7R-review/
As a quick summary, he notes the Sony sensor is superior to that used in the Canon. With the Sony image file, he can extract details in the shadows which are obscured by noise in the Canon files. From his tests, he encourages Canon scenic photographers to adopt the a7R with the Metabones adapter and use their best Canon lenses with this setup.
I've been impressed with the a7R and am starting to settle on a working outfit which I take with me everywhere.
For general walk about photography, I use the a7R with the 28-70 mm FE lens and a 20 mm f/2.8 Nikon wide angle mounted on a Voigtlander adapter.
On those occasions when I suspect I will need a longer reach, I will take a 100-300 mm APO Minolta zoom with the LAEA-4 adapter. This makes for a versatile lightweight outfit. Its main shortcoming is that I lose the SteadyShot feature on the 100-300mm. I compensate for its lack by raising the ISO--I've used ISOs as high as 8000 and have managed to take reasonable pictures by using careful handholding techniques. Initially, I thought that departing this far from the ISO 100 would result in unacceptable image quality but this has not been the case. Autofocus works very nicely with this lens. The 20 mm lacks stabilization, but with a lens this wide, I do not find it a handicap and the lack of autofocus is not much of a problem. I set to the lens to the hyper focal distance and with f/11 everything from infinity to about 2.5 feet is rendered sharply. It is nice to be able to use a depth of field scale, again!