Essentially, both cameras will produce great images regardless of the scenario. And well they should, as they represent the best that each manufacturer has to offer at this point. Professional photographers will probably end up sticking with the system that they’re already invested in, but if you’re starting fresh then we’d say it comes down to how you intend to use the camera.
If you’re intending to do a lot of portraits, the A7R III’s eye-detect AF will help vastly there, as will the lower weight of the entire system. If you’re looking primarily to capture action shots, then the D850’s more accurate 3D Tracking system will give it a slight edge.
Where the A7R III has a definite advantage is in silent shooting – the D850’s silent shooting is only via Live View mode, and taking stills via the rear LCD on a heavier camera doesn’t really make for great handling. If you’re a photographer who splits time between stills and videos, the A7R III’s Fast Hybrid AF, support for various log formats, and in-body 5-axis stabilization will also make it stand out in terms of video shooting. The improved battery life of the new battery also helps the camera overcome a common complaint of mirrorless cameras, as 500 shots on a single charge is certainly adequate for most uses. Add to that the slightly lower price (S$4,699 vs. S$4,999) and we have the winner of this shootout – the A7R III.