For me, they're so different that it's hard to compare them. Obsidian is a great tool for linking, searching, querying, and visualizing sets of thoughts. It happens to have a text editor, but that's not the brilliant part about it. Obsidian's editor feels to me like a method to get data into the note database. Also, Electron.

iA Writer is a one the best text editors I've ever touched. It happens to have tools for linking and searching notes, but that's not the brilliant part. Writer's data management feels to me like methods to navigate around to the note I want to writing. Also, it's a "Mac-assed Mac app", looks and feels exactly like it belongs on my desktop, plays nicely with all other Mac apps, and couldn't be faster or more responsive if it were hand-coded in assembler.

If I want to store and search notes, I'd probably pick Obsidian (although I'm currently using Bear Notes for that). If I want the best writing environment I know of, I'll reach for iA Writer every time.

I use both, and won’t exchange one for the other. Obsidian is a knowledge management tool. iA is on the other hand is a writing tool. I compose my blog posts there, esp. its writing aids are very helpful.

While obsidian is getting more and more of my knowledge, all focused post authoring is happening and will happen in iA.

Not who you asked, but I like and own both, but have found myself moving to Obsidian more and more. I'm not entirely sure why. IA writer now has document linking and I love the typography. I like the editor better.

I think there is something that makes file organization easier in Obsidian. I have a plugin that creates a new note for each given week. And it feels easier to navigate a bunch of files without the descriptions/first few lines of the doc.

EDIT: And this thread has encouraged me to second guess myself about some of these things and there's a setting to remove descriptions in the library. Maybe I'll try picking IA Writer back up!