I think the main difference would be that if you want to play with mods, you can absolutely do so with either version, but it is easier through Steam. I believe multiplayer still works just fine across Steam, GOG and Game Pass versions. And of course playing on Steam will unlock Steam achievements vs Xbox achievements on the Game Pass version. The game itself is identical on both platforms as far as I know.
Worth mentioning that the Game Pass version does not offer any cross platform benefits, if you start a save on PC with the Game Pass version, it doesn't sync the save to the Xbox console or allow playing multiplayer with those on the console. Some other Game Pass games do allow this so I figured I would mention it.