When comparing StackEdit vs nvALT, the Slant community recommends StackEdit for most people. In the question“What are the best Markdown editors for OS X?” StackEdit is ranked 3rd while nvALT is ranked 14th. The most important reason people chose StackEdit is:
StackEdit works within your browser. You need internet access to connect to the website, but once it's loaded, the site does not require an internet connection - you will be able to edit and save files locally. Additionally, you can use [Fluid](http://fluidapp.com) to turn it into a native desktop application.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Works online and offline
StackEdit works within your browser. You need internet access to connect to the website, but once it's loaded, the site does not require an internet connection - you will be able to edit and save files locally. Additionally, you can use Fluid to turn it into a native desktop application.
Pro Supports various Markdown flavors
Supports standard Markdown and Markdown Extra.

Pro Simple workflow
It is very easy to toggle between the preview and the editing windows. The black makes it a very focused writing environment. Of course it's possible to change that if needed.

Pro Great when in full-screen
Pro No need to install additional software
StackEdit works directly from the browser, there's no need to install additional software as long as you have a web browser installed on your computer.
Pro Real-time preview
The preview shows in a collapsible pane on the right hand side.
Pro Instant publishing
StackEdit allows pushing a document directly to a list of publishing or file storage platforms or any SSH server.
Pro Multiple export options
Can export to .txt, .html and .pdf.
Pro Syncs via Dropbox and Drive
Files can be synchronized through Dropbox and Drive.
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 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 Free

Pro MultiMarkdown 3 support
Has built-in support for MultiMarkdown 3.
Pro Right to Left support
Cons
Con Saves files in browser's local storage
Local storage is limited and if the browser crashes the text can be lost.
Con Cannot be accessed while being offline
Even though the editor itself works offline, you need internet access to open the website and the editor.
Con Lacks a good integrated spell checker
Uses the built-in browser spell checker which may not be as good as spell checkers other editors have. Depending on the browser, of course.
Con Publishing to GitHub requires giving write access to repos
In order to publish documents to GitHub, StackEdit requires writing access to your repos, something which many people may not be comfortable with.
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.
