When comparing StackEdit vs Checkvist, the Slant community recommends StackEdit for most people. In the question“What is the best cross-platform note-taking app?” StackEdit is ranked 28th while Checkvist is ranked 42nd. 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 As simple or as complex as you need
Checkvist hits a sweet spot between minimal and complexity. It gives you a host of options and extras to use on your lists, but you can completely ignore them if you don't need them and keep it very simple. This makes Checkvist a one-stop shop, regardless of what type of 'list' you need.
Pro Tasks can be managed with custom tags
Pro vim-like commands
Pro Convenient search by text or tag filtering
Pro Converts incoming emails to todos - this is a way to enter tasks from anywhere.
Pro Great search
Search filters help you search for tasks by tag, due date, assignee, etc.
Pro Very flexible
To-do lists often need to be grouped and regrouped for clarity. The outline-based nature of Checkvist makes it easy to move things across categories, to increase and decrease priority, etc.
Pro Smart parsing of items
Parses markdown, html links, hashtags to become formatted text, clickable links, and tags, respectively.
Pro Generous set of free features
Checkvist offers many free features if you are using this individually.
Pro Supports comments on tasks
Any item can have a free-text comment, which can be shown or hidden as desired.
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 Limited capability in the android app
Checkvist is primarily a web interface.
