No one does image quality tests anymore in review sites. Here’s an attempt to look at differences between 4K image quality of Nvidia and AMD cards. The review needs to be more through (and more profession imo) but at least it’s a start.
Linux has proper dithering for Nvidia hardware so it’s not as bad as Windows. Either way looks like Nvidia’s still has ground to cover to equal AMD in image quality and AMD has ground to cover to equal Nvidia’s performance.