Pretty different systems, bazzite is based on ublue and uses an image based system for it's immutability. You can add packages by building additional layers where steamos immutable only lets you modify the user space with changes that persist over updates. That makes Bazzite and other ublue distributions much more flexible for things other than just gaming.

For example, I love to use my bazzite steam deck for RTL-SDR radio and that needs a driver installed as a system package :)