StackEdit was designed as an online Editor with extensive Markdown support. In contrast, Classeur was designed to be a holistic writing and collaboration environment. Its focus lies on simplifying the creative process of writing and providing the user with functions for sharing his writing with others to receive constructive feedback. Similar to StackEdit, Classeur uses Markdown as its native language of composition.
StackEdit is no longer being developed. Thus, Classeur is the true successor of StackEdit.
We are working hard to provide the same level of features as StackEdit. For more information, please see our Roadmap for milestone 2.0.