The Xbox Series X will give you over the Xbox Series S:
* HD and UHD Blu-Ray.
* Super Sampled Xbox, Xbox 360, Xbox One, Xbox Series X games where available. (I.E. Games will look as crisp as possible.)
* Better texturing.
* Better texture filtering.
* More storage.
* Better Anti-Aliasing.
* Better reflections, lighting and shadowing due to the potential extra Ray Tracing resources.
* Potentially better/higher framerates. (Varies by game of course.)
Just because a console is built for 4k, doesn't mean you don't get benefits on a 1080P display, once the Series X drops... My Xbox One X will be going into the games room on a 1080P 75" display and it will definitely look better than the base Xbox One.
The Series S *can* do 4k, but developers are very unlikely to leverage that.
Very true almost everything on the post. But the last point well, perhaps MS can use their smart delivery tech for at least BC game to be 4k on Series S. And sure some Indies for next gen as well.
Microsoft has already stated that the Xbox Series S will be using the Xbox One S variant of games, not the enhanced Xbox One X variant, that includes the Backwards compatible titles.
The only games that will operate at 4k will be those games with an uncapped dynamic resolution that will scale up that high or a developer has released a patch for.