Rename "broadcast scheme"
I don't think any of those schemes are fundamentally different. The fundamental difference in threat model (and in implementation) is can we start sending message before everybody a reach same view of the room or not.
Original mpOTR doesn't allow it and it is much simpler in implementation and we have security proof for it. My guess is that it doesn't make a huge difference in user experience in majority of xmpp usages. It is also not hard to tweak the code and algorithm to fall back to the different view scenario, that is why I prefer to go with that.