Obsidian and Downer both work with plain Markdown files on your own disk, so there’s no lock-in either way. They solve different problems.
Obsidian is a home for your notes
Obsidian is a notes system. You keep a vault of Markdown files and link them together, with backlinks, a graph of how your ideas connect and Canvas for laying them out. Thousands of community plugins add almost anything you can think of. It runs on Mac, Windows, Linux, iPhone, iPad and Android. It’s free for personal use, with paid add-ons: Sync to keep a vault on all your devices, from $4 a month per user billed yearly, and Publish to put notes on the web.
For linking, mobile access and syncing between devices, Obsidian is the stronger choice.
Downer is a reader for any file
Downer doesn’t ask you to set anything up. Double-click a README in a project folder, a notes file someone emailed you or a LaTeX paper, and it opens as a finished page almost instantly. Mermaid diagrams, math and papers are drawn without plugins, in 15 themes. Files keeps your recent documents, and opening a folder gathers its Markdown into a collection. Press Space in Finder and Quick Look shows the same page.
The short version
If you’re building a second brain, choose Obsidian. If you want the fastest, cleanest way to read the Markdown scattered around your Mac, choose Downer. Plenty of people will want both.