When comparing MacDown vs Coda 2, the Slant community recommends MacDown for most people. In the question“What are the best developer tools for Mac OSX?” MacDown is ranked 4th while Coda 2 is ranked 19th. The most important reason people chose MacDown is:
MacDown is a free and open source editor influenced by [Mou](http://25.io/mou/). It's released under the MIT license.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Free and open source
MacDown is a free and open source editor influenced by Mou. It's released under the MIT license.

Pro Real-time split-screen preview
MacDown's main view is split into two panels. The user types on the left and the Markdown is rendered on the fly in the right panel. This helps users to better understand the way they are formatting their document.
Pro Markdown previews can be customized with CSS
You can use a CSS file to customize the rendered output and the file preview you are working on will display the rendered Markdown with the custom CSS styling on top.

Pro Supports syntax highlighting in fenced code blocks
MacDown has syntax highlighting support for various languages when writing code in fenced code blocks.
Pro Good auto-completion
MacDown has a good built-in auto-completion engine for Markdown symbols.

Pro Support for GFM
Pro Ideal for day-to-day programmers' work and MarkDown novices alike
Using MacDown for the notorious README.md use case gets you going without reading any manual or requiring any configuration values. Think of it as a sort of TextEdit for MarkDown files. Thus its shortcomings - neither powerful nor versatile - turn out to be a PRO for novices trying to jump on the MarkDown bandwagon. Open its help and you'll immediately find yourself editing the MacDown's MarkDown help file, a MarkDown primer with some MacDown menus and configuration added.
Pro Linking between pages
Unlike a few other editors, MacDown lets you link between markdown pages.
Pro Tool bar with most used markdown shortcuts
This is especially useful for Markdown novices
Pro Real IDE with all the benefits
Coda 2 comprises all you would expect from an IDE: it supports multiple languages (including all the standards); it performs autocomplete of project names, as well as language functions; it supports SVN and GIT; it has good support for plugins (or you can write your own); it has a configurable editor; and it has a built-in preview.
Cons
Con Not very versatile
MacDown is not very powerful or versatile. It's not customizable or extendable. This is what makes it so simple, but it's not for people who want more from their tools.
Con Frequently fails to update the display and/or flat out hangs
Must often restart MacDown.
Con The Markdown preview is rather heavy on the CPU
The Markdown preview needs a lot of resources to keep rendering on-the-fly after each keystroke. A single keystroke in the editor panel may trigger up to 5 seconds of max-CPU usage.
Con Defective UI
There are several things in Coda that simply don't work, and never have. For instance, the root directory for your local and remote files is simply not honored. For every project, you can specify the root directory for its files. But when you open the project in Coda, it doesn't go there. The file browser just shows whatever the last directory was that you were using, and will write files to the wrong place. Thus, it defeats the purpose of setting the home directory in the first place.
Also, splitting the editor doesn't work. If you've done any programming, you know how important it is to be able to view two files simultaneously. Coda fails to do this, with a bizarre insistence on making the two panes dependent on each other.
Con Support for Mac and commercial use only
Coda 2 is only available on Mac (even though that does make it a native app, meaning its much faster). Coda 2 costs $99 after the 30 day free trial is up.
Con No XDebug
If you also write PHP, there's no XDebug support available.
