Here are some comparisons to get you started:
Note that these images were processed in their respective brand's RAW converter (Sony-blessed Capture One Express for Sony was used for the a7R III results). With a sharpness setting of one (our current Nikon and Canon standard), the Canon appears undersharpened.
Also note that I updated this tool to permit additional scenarios, including not-full-stop ISO settings for some cameras. The D850 (and Z 7) results have ISO 32 (extended) and ISO 64 included and (I'm sure you have been anxiously awaiting this) ISO 40000 results for the EOS R are now included. Please report any problems found.
Please share these results with your friends with Nikon-based kits!
Great deal: Save nearly $330 on the D850 in 10-condition at B&H used.