OP didn’t ask about the Series S, but One X vs Series X.

One X, while better GPU and more RAM, doesn’t support next gen titles (of which there are a ton of that OP specifically asked about latest Forza) it also still runs the One/One S CPU which might be why it doesn’t do next gen.

Series X will impress, and more so if your display can match all the features (DV, HDR10, ALLM, VRR, 120Hz, etc)