When comparing MultiMarkdown Composer vs Caret, the Slant community recommends MultiMarkdown Composer for most people. In the question“What are the best Markdown editors for OS X?” MultiMarkdown Composer is ranked 12th while Caret is ranked 25th. The most important reason people chose MultiMarkdown Composer is:
Not many popular Markdown editors support MultiMarkdown syntax.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Multimarkdown support
Not many popular Markdown editors support MultiMarkdown syntax.
Pro Multiple export options
You can export to HTML, PDF, LaTeX, RTF, OpenOffice, Microsoft Word, ePub, OPML.
Pro Table of Contents support
Generates a table of contents based on the headers in the Markdown file being worked on.
Pro Support for elastic tabstops
Elastic tabstops are used to align text automatically during writing. When there's a change in the text, MultiMarkdown Composer automatically aligns all lines both on the beginning and end of them.
Pro CriticMarkup support

Pro Optional synchronized scrolling
Hold a key down and get synchronized scrolling.
Pro Fast syntax highlighting
Pro Highly customizable
Pro Counts words, characters, lines
Pro Can show invisible characters
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 Table of contents gets out of sync sometimes
If you keep the auto-generated table of contents open while you work it gets out of sync, meaning items in the table of contents (at the end) disappear until you restart the program.
Con The way the preview shows images is inconsistent
The preview is a bit inconsistent with images. Sometimes it shows the image correctly, sometimes it shows a blue question mark.
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.
