The best comparison will come if/when the DRM gets cracked and the original source streams become available.
I wonder if the source streams will even be accessible, though. I know that for Netflix offline downloads, the copy you download isn’t the source stream, it’s a still-heavily-compressed downloadable version. I wonder if that’s what people will be able to save to their hard drives on Disney+ as well.
either way, the source will be made available sooner rather than later when the UHDs are officially released sometime next year.