Camera Labs has posted RAW image comparison of new Sony a7RII (Amazon, B&H Photo Video, Adorama) and Canon EOS 5Ds R (Amazon, B&H, Adorama). Using a third part lens on both camera: Sigma 50mm f/1.4 DG HSM Art lens. ISO at 100.
A handful of photographers who make huge tonal adjustments in RAW may prefer Canon for its uncompressed RAW files, and I certainly wish Sony would offer an uncompressed RAW option just to satisfy a complete market, but personally speaking I’ve never had any issues with retrieving reasonable highlight or shadow details from Sony files. I’m satisfied the A7r Mark II and EOS 5DSr share essentially the same real-life resolving power.