When comparing Byword 2 vs MacDown, the Slant community recommends Byword 2 for most people. In the question“What are the best Markdown editors for OS X?” Byword 2 is ranked 2nd while MacDown is ranked 5th. The most important reason people chose Byword 2 is:
Byword de-emphasizes the syntax itself while emphasizing its effects. It appropriately adds style, like italic and bold, to text that's designated by markdown and dims the syntax so it does not get in the way of comprehension. Additionally, there are commonly used hotkeys (⌘b, ⌘i, etc) that can be used to apply style without having to know the syntax or having to type it out each time.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Great Markdown support without being intrusive
Byword de-emphasizes the syntax itself while emphasizing its effects. It appropriately adds style, like italic and bold, to text that's designated by markdown and dims the syntax so it does not get in the way of comprehension. Additionally, there are commonly used hotkeys (⌘b, ⌘i, etc) that can be used to apply style without having to know the syntax or having to type it out each time.
Pro Available for all Apple devices
Byword is available on a Mac, an iPhone and iPad.
Pro Extremely simple
Byword is as simple as Markdown editors can get. Even though Markdown itself is not hard, Byword offers assistance to make it even easier with very little configuration required.
The UI is minimalistic and un-intrusive, the syntax auto-completion is excellent, and there are multiple small but helpful features that make writing Markdown with Byword a breeze.
Pro Syncs using iCloud and Dropbox
Documents can be synced using iCloud or Dropbox from within the editor for use across all devices.
Pro Direct publishing to web platforms
Byword allows users to publish their markdown files directly to web platforms such as Wordpress, Tumblr, Blogger and even Evernote.
Pro Word count support
Has a built-in word counter.
Pro Multiple export options
You can export to HTML, PDF, RTF.
Pro A dark theme is available
For people who enjoy working during the night, there's a dark theme available that reduces strain on the eyes.
Pro Includes features that speed up writing
For example, while authoring a bulleted list hitting return automatically prepares a new bulleted line.
Pro Live update support
Byword has a "Preview Markdown" option where it does exactly what it promises. It immediately compiles and displays the rendered Markdown file, making it easy for the user to understand how they are actually formatting their document.
Pro Supports Multimarkdown
Byword supports all the table types in Multimarkdown including table captions. MathJax support is added via metadata at the beginning of file was well as support for various diagrams. Table of Contents is also supported.

Pro Supports iCloud Drive and Handoff
One of the few (well, Ok, only for far) that has been updated for Yosemite features. Jan 2015.
Pro Updated frequently
Byword gets frequent updates which add new features or fix existing bugs.

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
Cons
Con No support for code blocks
I need my markdown editor to support code blocks like
def foo(bar, baz):
qux()
This one does not.
Con Publishing requires additional in-app purchase
The feature to publish Markdown files to web services costs an additional $5 of in-app purchase.
Con Expensive
Con Limited customizability
One of the things that makes Byword so easy to use is the fact that it needs very little setup and option-tweaking. However, that severely limits the ability to customize the editor itself. For example, there are only two themes available, a light one and a dark one.
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.
