Bazzite supports Nvidia graphics cards, which is a major difference if it affects your hardware.

It also uses the same kernal optimizations for gaming that Glorious Eggroll maintains for CachyOS, which I doubt SteamOS utilizes. It's not a huge increase to performance and they slow down write speeds to disk as a tradeoff, but it'll often get you a couple extra frames in games.

Bazzite also uses Bazaar instead of Discovery as its application manager. They're both based on FlatHub, but you might have slight variations in available software.