Chiming in on an old thread...
I bought an Xbox Series S the other day. I wanted the Series X but those are really difficult to find and patience is not my thing. I read a little bit about the two consoles and decided that the S would be good enough and for most games I'm not sure I can tell a difference between true 4K and 1440 upscaled 4K. HOWEVER, on PGA Tour 2K21 I can absolutely, 100% tell the difference. It's almost night and day.
A little background first (sorry for the longish post). We just moved into a new house a few weeks ago which prompted me to add a TV so now I have two main rooms in which we watch TV (downstairs family room and upstairs family room). I sold my old TV when we moved out of our last house and bought two new TVs for the two family rooms. I put a 55" Sony 900H in the upstairs room and a 65" Sony 900H in the downstairs family room. I've got the Series S running on the 55" Sony 900H and the Xbox One X on the 65" Sony 900H so I can pretty easily see the differences in the video quality between the two systems.
For games like Forza Horizon 4 I'm not sure I can really tell a difference. But for PGA Tour 2K21 there are jaggies EVERYWHERE on the Series S. It's most noticeable on the green grid lines. On my One X the green grid lines are smooth as silk and on the Series S there are stair steps everywhere.
Additionally, the trees and grass textures on the One X are gorgeous whereas on the Series S they look a little garbled and fuzzy. I may not have noticed it if I had never played the game on the One X before playing it on the Series S.
I kinda wish I had just waited to get the Xbox Series X. I would've been happier but oh well. I don't play enough games for it to really matter I suppose and I'm still super pleased with the One X on my bigger TV downstairs.
I'm glad you shared this report Mebby, I've been wondering about this game on the Series S. I've asked in multiple forums, official and otherwise, but no one has been willing or able to answer the question about resolution as you just have.
I strongly suspect that what you're seeing on the Series S is pure 1080p, being upscaled to 1440p (by the Series X), and then upscaled again to 4K by your your TV. If that's indeed what's happening, it's going to look like upscaled 1080p, which in my experience is a far cry from native 4K.
PGA TOUR 2K21 is said to be "backward compatible" on the Series S and X. The vanilla "backward compatible" classification means that it's not going to get any additional enhancement or optimization. In the case of the Series S, that means it's going to emulate an XBox One S in backward compatibility mode. Do you see any AUTO HDR at work on the S version? It makes a noticeable difference on the Series X, IMO.
Even though PGA TOUR 2K21 looks a little better on the Series X, and the controller inputs feel less laggy, that 30 FPS frame rate cap still has its foot on the throat of this game. Even on a powerhouse like the Series X, being throttled by that cap just takes the life out of it for me, having grown accustomed to playing it at 60 FPS on the PC.