Both are based on Fedora but the main difference is that Bazzite is immutable, while Nobara is not. What you prefer, is highly subjective.

If your question is aimed at performance, I have no hard data on that and it will depend on your hardware of course, but they are probably quite similar, since the base is the same.

That’s not too hard a question for me, I’ve been using the same DE for years: KDE