When comparing Elephant vs StackEdit, the Slant community recommends Elephant for most people. In the question“What is the best cross-platform note taking app?” Elephant is ranked 14th while StackEdit is ranked 26th. The most important reason people chose Elephant is:
Only basic features are added to Elephant so users aren't overloaded with the interface.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Only the essentials
Only basic features are added to Elephant so users aren't overloaded with the interface.
Pro Files are stored locally
Instead of moving files over the cloud, files are stored locally for extra privacy. For syncing, any cloud service can be used.
Pro Open source
All features can be used or edited at will for free.

Pro Markdown supported
This can be useful when formatting files in HTML.
Pro Files can be uploaded by drag'n'drop
Pro Highly compatible
Elephant is compatible with Mac OS, Windows, and Linux.
Pro Active development
The developer has replied, and added many features which users have suggested in the comments on the website.
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 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 Multiple export options
Can export to .txt, .html and .pdf.
Pro Supports various Markdown flavors
Supports standard Markdown and Markdown Extra.
Pro Instant publishing
StackEdit allows pushing a document directly to a list of publishing or file storage platforms or any SSH server.
Pro Syncs via Dropbox and Drive
Files can be synchronized through Dropbox and Drive.

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
Cons
Con No mobile app
No mobile apps (can use common sync tools to edit plain files, though).
Con Light on features
While not labeled a beta, Elephant is a new note program and doesn't offer all the bells and whistles.
Con No WYSIWYG editor
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 Saves files in browser's local storage
Local storage is limited and if the browser crashes the text can be lost.
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 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.