When comparing Mou vs Caret, 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 Caret is ranked 25th. 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
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Pros
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.
Pro Single-pane hybrid interface
The single pane formats your MD as you write it but retains the markup so you can see what's going on.
Pro Has a heading navigator
Hit ⌘ G to see document titles in a hierarchical view, type and use the arrows to navigate.
Pro Has a file manager
Hit ⌘ T to browse current folder, navigate the file system and manage your files and folders, type to search, right click to create / move / rename / delete items.
Pro Supports multiple cursors / selections
Make ten changes at the same time, not one change ten times - lets you edit multiple lines at once, perform quick and selective find and replace, etc.
Pro Supports context commands
Hit "Enter" to fix spelling, convert list to ordered/unordered/task, convert reference link to inline, jump to reference link definition, jump to footnote definition, visit links, etc.
Pro Supports auto-completions
Auto-completion for paths of links and images, keywords in fenced code blocks, emoji, etc.
Pro Offers inline preview for LaTeX math expressions
Renders LaTeX math expressions as soon as you have finished writing them.
Pro Supports "select more" / "select less" commands
Hit ⌥ ↑ to extend the current selection to the next biggest element and ⌥ ↓ to shrink the selection.
Cons
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.
Con Not free
No longer free, try and buy
Con The shortcuts don't toggle formatting
Pressing cmd+B, for example, will begin a bold segment of text, but pressing it again doesn't end that bold segment, it retroactively turns the boldness off. This interrupts writing flow tremendously.