They're not incredibly different, but they're different enough that they're not closely comparable -- in my experience, anyway.

From what I can tell, having used both for a few years, is that Scrivener is excellent for saving hundreds/thousands of textual notes, then very quickly refinding them, and offering an elegant word processor for writing both notes and one's final piece.

It's not great at syncing between different devices. It has a mobile app, but it uses Dropbox for syncing with the desktop versions and -- my experience and in the experience of people close to me -- it is super crash prone, and those crashes seem to lose data. So, I wouldn't touch their mobile app with a ten-foot pole.

Notion isn't as good at quickly retrieving search results; you could write hundreds/thousands of notes but you wouldn't find it as easy to re-find things. So I wouldn't use it to organize a massive amount of note-taking.

But -- it works and syncs very well across all devices; and it is extremely well designed for making formatted lists. So I find it very useful for precisely that: Keeping lists of ideas.