Broadcasting

1) Broadcast is an easy way to get two sprites to converse without having to use a ton of wait buttons.  It makes it less confusing and smoother.
2) I would use timing when two sprites are simply talking without an interruptions.  It would be clear how long each wait would be since you know how long someone is speaking.  I would use broadcast when more things are happening, like costume changes and background changes.  These types of blocks don't say how long they are so it would just be more simple to use the broadcast.