@smallcircles @helge @manyfold
For domain-specific projects like Manyfold - yes, it's pragmatism vs best practice. You get better interop if you choose Note.
For general purpose implementations, it's both (and I consider microblogging services general purpose, similar to RSS feed readers). Property based approach works really well for processing incoming Create activities. If object is attributed to an actor and has content, microblogging application should display it as a post, I don't see any reason to do otherwise.
In other situations, however, type based approach is preferable: activities, tags, attachments.