M and AP are started with different purposes. M for instant messaging and AP for social networking.
Therefore their vocabularies differ. For example, M talks about events, while AP talks about Activities. M has a concept called a virtual 'Room', while AP I am not sure (may be lists?).
Both M & AP puts E2E to use on both client-server and server-server API with different algorithms ofcourse.
AP has become a w3c standard while M hasn't.