Gnome is more stable in my experience. The base Gnome without any extensions is rock solid and just works without a hitch. Problem is that it’s bland as hell and not very practical. Most of the popular extensions to make the DE are also pretty solid and reliable, as gnome’s extension API doesn’t really allow them to break anything in any spectacular way. Plasma allows better and more powerful extensions, but they also tend to have a bigger effect on the DE’s stability.

That said, I fucking hate how Gnome devs handle the extension API. Every update to Gnome disables all the extensions even if there’s no breaking changes that would cause them to stop working. You gotta either manually edit their manifest files to trick Gnome into thinking they’ve been updated to the latest DE version, or you gotta wait for the devs to update. Every now and then they do release a breaking change that does break things too, and things get annoying. I used to maintain a relatively popular-ish, very simple Gnome extension. But eventually I got sick and abandoned because nobody’s got time to deal with Gnome’s extension API. I had to rewrite some basic shit for no good reason one too many times.