I've invested in Mobiledoc (mostly because it was for two ember apps) and it's mostly been fine, but is there any reason why I shouldn't be moving to ProseMirror? Seems to be a lot more powerful and have things like collaborative editing and track changes as well.
Just my experience: I had a rough time with ProseMirror trying to use it before it was released. It didn't use modern JS modules for a long time, the source code was a mess as far as I could tell, and JavaScript variable errors would surface with every version. Maybe the author has overcome these issues, but I'd be inclined to go with a different editor. Also, ProseMirror's architecture is extremely close to DraftJS,…
We then spent 2-3 weeks getting all of the formatting, tooltips and key mappings to handle some basic formatting (bold, italics, underline, bullet lists, numbers). Eventually, once there are more examples on the web of more complex formatting for the community I think it will become much easier to use.
It's the first editor I've used that felt like it made sense at a conceptual level but implementing "simple features" can feel very hard at times. I don't think that's a knock on the library though. There's a lot to learn and understand and it just took us a lot of time to get it all.