When comparing Writedown! vs MWeb, the Slant community recommends MWeb for most people. In the question“What is the best Markdown editor for OS X?” MWeb is ranked 17th while Writedown! is ranked 34th.
Ranked in these QuestionsQuestion Ranking
Pro Completely free
This project is distributed under LGPL
Pro The first free markdown editor in Mac App Store
Pro Excellent support of LaTeX equations
Pro Small but powerful
It has file tree, can post article, can upload pictures, can render in pretty preview themes, and is small and fast.
Pro Supports publishing to popular web platforms
MWeb can publish Markdown files to various web platforms including (but not limited to) Wordpress, Metaweblog, Evernote, Blogger and Tumblr.
Pro Syntax highlighting in fenced code blocks
MWeb has syntax highlighting for some popular programming languages inside fenced code blocks.
Pro Drag & drop or paste photos to the editor and display
Supports adding pictures to a Markdown file either through drag-and-drop or by pasting it.
Con Lack of features
The app only supports simple left preview/right editing view, nothing else.
Con No free version
MWeb costs $14.99 and is available on the Mac App Store. There's a free trial available but it's only for 14 days which may not be enough to make a good evaluation of the software before buying.