I have both a steamdeck on SteamOS and a pc on Bazzite and they both work exactly the same. They both run literally the same steam UI and both run flawlessly so I couldn’t even tell you which one I’m using if you just showed me a screen and controller.
There is difference in the desktop part. SteamOS has KDE as gui and Bazzite you can pick Gnome. Another major thing is that Bazzite is better prepared for desktop use - it has many ways how to instal packages and dev tools. I run bazzite and webdev on it comfortably.