When comparing MultiMarkdown Composer vs nvALT, the Slant community recommends nvALT for most people. In the question“What are the best Markdown editors for OS X?” nvALT is ranked 10th while MultiMarkdown Composer is ranked 12th. The most important reason people chose nvALT is:
nvALT has a simple text box at the top (similar to that of a web browser) and when you start typing inside it, nvALT immediately displays a drop-down box of all the notes that contain the text you are searching for. The more you type, the more the search is narrowed down.
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 Powerful search syntax
nvALT has a simple text box at the top (similar to that of a web browser) and when you start typing inside it, nvALT immediately displays a drop-down box of all the notes that contain the text you are searching for. The more you type, the more the search is narrowed down.
Pro Lightweight
Pro Free

Pro MultiMarkdown 3 support
Has built-in support for MultiMarkdown 3.
Pro Powerful note-taking features
nvALT is not only used for simple text editing but also as a powerful tool for taking notes, TODOs, phone numbers and every other piece of information that can be stored in a plain-text file.
This is because nvALT is not designed only as a tool to write text, but also as a powerful search and organize tool for plain-text files.
Pro Right to Left support
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 very powerful as far as text editing goes
nvALT's strength is its search feature. It has some basic text-editing features such as cut, paste, find-and-replace and so on. But it doesn't have any advanced text manipulation features available in other editors.
