When comparing Emacs vs Mou, the Slant community recommends Mou for most people. In the question“What are the best Markdown editors for OS X?” Mou is ranked 8th while Emacs is ranked 18th. The most important reason people chose Mou is:
Mou has word auto-completion (accessed via 'esc') for English words, text transformation such as conversions to uppercase, blockquote or an h1 heading, HTML entities (such as <’s, &’s and spaces) that can be added anywhere in the text and customizable keyboard shortcuts for everything allowing for efficient use of the software's capabilities.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Keyboard-focused, mouse-free editing
Emacs can be controlled entirely with the keyboard. While true, I often find the mouse and menus handy for those lesser-used commands. An aide-memoir.
Pro Total customizability
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect (Emacs Lisp). A robust list of existing Lisp extensions include the practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the sublime (chess, Eliza).
Pro It's also an IDE
You can debug, compile, manage files, integrate with version control systems, etc. All through the various plugins that can be installed.
Pro Free
Licensed under GNU GPL.
Pro Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Pro Works in terminal or as a GUI application
You can use Emacs' command line interface or graphical user interface.
Pro Great documentation
With 30+ years of use the Emacs documentation is very thorough. There are also a lot of tutorials and guides written by third parties.
Pro Ubiquity
Fully compliant GNU-emacs is available on many platforms, and they all understand .emacs configuration files.
Pro Lisp customizations
With lisp customization, any behavior of Emacs can be changed. Update with pre-release patch can be also applied without recompiling the whole Emacs.
Pro dabbrev-expand (Alt-/)
Dynamic word completion.
Pro Provides org-mode
Advanced planning and publication which can start as a simple list.
Pro Mini buffer
You can pass complicated arguments in the mini buffer.
Pro Enormous range of functionalities (way beyond simple "text editing")
Through its programmability, a very broad range of functionalities can be integrated in emacs, turning it even into a "single point of contact" with the underlying operating system.
Pro Rectangular cut and paste
Emacs can select rectangularly.

Pro Vi keybindings through Evil mode
Evil mode emulates vim behaviors within Emacs. It enables Vi users to move inside the Emacs universe.
Pro Has been widely used for a long time
The first verion of Emacs was written in 1974 and GNU Emacs in 1984.
Pro Visual selection and text objects with Evil
Evil is an extensible vi layer for Emacs. It provides Vim features like Visual selection and text objects.
Pro Support multi-line editing, multiple frame, powerful paren, crazy jumping style
Review the "Emacs Rocks" video.
Pro Cross-platform
Works on Linux, Windows, Macintosh, BSD, and others.
Pro Integrates planning in your development process
You can jump straight from your org-mode files to programming tasks - and back - and build a seamless workflow.
Pro Helm plugin adds even more power to Emacs
Powerful commands, search, and more with the Helm plugin.
Pro Versatile
Emacs is great for everything.
Pro GTK+ widgets support
Since version 25 you can run GTK widgets inside Emacs buffers. One of these is the WebKitGTK+, which allows the user to run a full-featured web browser inside Emacs with JavaScript and CSS support among other things.
Pro Excelent tutorial to get you started
The tutorial you are presented with at startup shows you exactly what you need to get started and teaches you how to use the built-in help yourself later.
Pro Interactive Shells
Emacs has a number of shell variants: ansi-term, shell, and eshell.
Pro Gnus
Managing several large mailing lists has never been easier using Gnus. The threading commands and the various ways of scoring articles means that I never miss important messages/authors, etc. A joy to use.
Pro eshell is cross platform
You can use the underlying operating system shell as a terminal emulation in an Emacs buffer. Don't like the default shell for your configuration? You can change it to your liking.
Pro Excellent Lisp editing support
Built-in packages make editing Lisp source code feel natural.
Pro use-package and org-mode
Missing some neural package that predicts actions
Maybe in the next release ...
Pro Advanced text editing
Mou has word auto-completion (accessed via 'esc') for English words, text transformation such as conversions to uppercase, blockquote or an h1 heading, HTML entities (such as <’s, &’s and spaces) that can be added anywhere in the text and customizable keyboard shortcuts for everything allowing for efficient use of the software's capabilities.
Pro Full syntax highlighting
Mou provides full syntax highlighting for Markdown. The highlighting can be customized manually or by applying a predefined theme.
Pro Has real-time split-screen preview
Mou is split into two columns. It displays raw Markdown on the left and formatted text on the right. Text on the right will update in near real-time as raw Markdown is written.
Pro Powerful search
Mou can do incremental ("find as you type") pattern matching search. For example, it can find all words that have x as the third letter followed by at least 4 more letters and a line break.
Pro Free beta versions
Mou is available as a free download up until the release of 1.0.
Pro Customizable
The whole editor can be customized using different color schemes and users can create own variations by editing CSS.
Pro Support for CJK characters
Mou supports CJK characters for writing Markdown in Chinese, Japanese or Korean.
Pro Export as HTML or PDF
Markdown can be exported as an HTML or a PDF file. Each export can also be styled by a custom CSS.
Pro Customizable keyboard shortcuts
Mou has keyboard shortcuts for all text manipulation actions allowing for highly efficient use of the software. These shortcuts can also be manually edited to better fit each individual workflow.
Pro Includes optional typewriter keypress sounds
The app includes optional atmospheric typewriter sounds that are triggered with each keypress. These can be enabled in the settings.
Pro Scriptogr.am & Tumblr integration via Dropbox
Mou allows writing and publishing to blogs from within the editor.
Cons
Con Learning curve is long
While it's better than it used to be, with most functions being possible through the menu, Emacs is still quite a bit different from your standard editor. You'll need to learn new keyboard shortcuts.
Con Sometimes the extensibility can distract you from your actual work
If I ever want to lose half a day, I'll start by tweaking my .spacemacs config file.
Con Keyboard combinations can be confusing for new users
For example, for navigation it uses the b, n, p, l keys. Which for some people may seem strange in the begging. However they can be changed easily.
Con Documentation is not beginner-friendly
Although lots of good built-in documentation _exists_, I have after four years of Emacs as my primary editor not figured out how to actually make use of it, and rely completely on Google / StackOverflow for help.
Con Hard customization
For customization, you need to learn Lisp
Con A lot of jokes in this serious software
Con Using Emacs on a new machine without your .emacs file
Con No longer maintained
The latest version was released in 2014, version 1.0 was announced for 2015 but was never released. The app is not compatible with macOS Sierra.
Con Only free until 1.0 release
Once 1.0 is released (still unreleased as of May 2017, release was planned for August 2015), Mou will cost $30. It can currently be pre-ordered for half the price.
Con Text is hard to read on a big screen in full-screen mode
On big screens legibility is pretty bad when entering full-screen mode because the text is not columned nor centered. On smaller screens though, legibility is quite good even on full-screen.
Con Doesn't work as well with non-monospaced fonts
When using a non-monospaced font, the text jiggles around. It can be very annoying since users may not always want to write with a monospaced font.
Con Switching live preview on/off changes the width of text area
Con v1.0 was promised to Indiegogo founders more than a year ago
Safe to say this is dead software at this point.
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