The Series S is more powerful: https://www.eurogamer.net/digitalfoundry-2022-steam-deck-vs-xbox-series-s-how-future-proof-is-the-valve-handheld

But the Steam Deck is a portable console, so the design considerations are different, so it's a bit of an apple to oranges comparison. On pure numbers, though, Series S will perform better. (Steam Deck is still awesome though 👍)