If I could only keep one for the rest of the gen, it would without doubt be the Series X. For just $200 you're getting 3x the performance if not more, double the storage, more ram, faster ram etc. It's just far more future proof.
With that said, I actually ended up pre-ordering both systems, the Series X for my main room, and the Series S for the second room or bedroom, or to sell or return.
As there aren't really any(?) new exclusive launch games for the XSS/XSX besides Falconeer, if Digital Foundry shows multi-platform games are very close between the PS5 and XSX, I might just keep the XSS as a Game Pass machine for now, sell my XSX, but buy another one again several months later for cheaper for the main room.