Marked 2 and Downer both show Markdown as a finished page on your Mac. They start from opposite ends, though.
Marked 2 is a previewer for your editor
Marked doesn’t edit. You keep writing in the app you like, and Marked’s window updates every time you save. It’s built for writers: readability statistics, proofreading aids, custom styles and export to HTML, PDF and Word. It also works with Scrivener, MarsEdit and others. It’s paid, with a trial from the developer’s site.
Downer is a reader with an editor inside
Downer opens a document and shows it almost instantly, with a map of headings, a command palette and find. Mermaid diagrams, math and LaTeX papers are drawn without any setup. When something needs changing, the editor slides in beside the preview, and Fix Formatting tidies messy Markdown in one step. Quick Look previews in Finder come with it.
The short version
If you write in a favourite editor and want a polished preview with export and writing analysis, choose Marked. If you mostly read Markdown, want diagrams and papers drawn properly, and like having an editor in the same window, choose Downer.