Typora and Downer both open plain Markdown files from your Mac, so there’s nothing to import and nothing locked in. They’re built around different moments, though.
Typora is built for writing
Typora’s signature is its editor. You type Markdown and it turns into formatting straight away, with the syntax hidden, so the page always looks finished. It adds export to PDF, Word, ePub and more, and it runs on Mac, Windows and Linux. It’s a paid app with a free trial.
Downer is built for reading
Downer opens a document and gets out of the way. Pages appear almost instantly, long documents get a map of their headings, and diagrams, math and LaTeX papers are drawn without extra setup. When you need to change something, the editor opens beside the preview with a formatting bar, and Fix Formatting tidies messy Markdown in one step.
Downer also adds Quick Look previews to Finder, so you can read a Markdown or Mermaid file by pressing Space.
The short version
If you live in your editor and need exports, choose Typora. If you open a lot of Markdown written by you or someone else, and want it fast, readable and free, choose Downer.