I think the difference comes in the deployment story. Prose seems targeted to a developer who would need to manage a site on their own or that's hosted on Github pages.
Instead imagine you have a Rails app on Heroku that has some marketing pages on it (homepage, faq, etc.)
Contextify seems like it could let you give a non-developer person access to modify those pages in Markdown and get the changes 'deployed' without having to do a separate loop of checking them in, and pushing the whole app back to the server.
If it's on node - it does some things to get+render markdown on the pages within the server. With their jQuery plugin, I _think_ they are loading everything in from the repo after the fact (which brings up a bunch of other issues ranging from screwing with your SEO to being down if Github is down to being perceivably slow to load).
SDKs stores the content from Github in a cache so that it doesn’t need to be loaded everytime. Refreshing a page with the JS SDK can indeed be a bit slow. However, with the Node.js SDK, with the cache, it becomes really smooth. You can manually load all files and all the following users will receive a cached version.
Contentify is not just a Markdown editor. It allows you and your team to work together on all the content of your webapp. At OAuth.io, we use Contentify to integrate contents that our non developers collaborators write using the Node.js/Javascript SDK. They can save as a draft (not published on the webapp) or as a release (available to public). Like that, developers can focus on UX and where the content will be while non developers can focus on wording.
