As someone else building a notes app, I went with CodeMirror because I enjoy the feature-set of the obsidian editor (which is CodeMirror), and I'm trying to emulate the features on that that I use the most, in addition to some more "experimental" features I'm currently playing with.

Personally, I really don't enjoy WYSIWIG editors when writing notes. It's just unnecessarily different compared to what I'm used to. Though I can see non-devs enjoying it more.