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).