iA Writer vs Obsidian
Both iA Writer and Obsidian are Markdown plain-text editors that can be used to link together notes.
iA Writer
iA Writer is geared mostly towards professional writers who want a clean and simple tool to create and organize their work. Like Obsidian, it also has a mechanism for linking notes together, though significantly less powerful one.
Obsidian
Obsidian is a lot more sophisticated in its note-linking abilities. You can easily build and navigate whole knowledge bases with Obsidian. That said, it's definitely more complicated and less aesthetically pleasing than iA Writer.
Fan of minimal note-taking apps? 🤔
Maybe Paper is what you're looking for.