I just don't understand what the point is of these things, same thing happened with the 6k Dell that was supposed to be a XDR replacement. Worse picture quality, worse specs, very little savings, and years late. Well in this case just one year late but still. And the Studio display is now available on sale sometimes too from retailers.
The Viewfinity S9 and U3224KB are general-purpose PC monitors and they are therefore designed around "most common denominator" feature sets whereas Apple creates bespoke displays designed to work first and foremost with Macs and macOS and their compatibility with other operating systems and hardware is, at best, an afterthought.
I prefer the glossy screen of the Apple Studio Display and since I would use a VESA mount, the better stand the Viewfinity S9 offers would stay in the box. So for an identical base price, I would go with the ASD. The one drawback is that I also need to use it with a Windows PC and from the threads I have read, that is a bit of a PITA with a PC with a Thunderbolt 3/4 connector and even more so with a PC that doesn't have a TB connector.
The Viewfinity S9 on the other hand "just works" with a PC and a Mac as it has both a Thunderbolt connector for the Mac and a (mini) DisplayPort connector and USB-C for the PC.