Using twitter to discuss the articles was a different experience since the conversation was more of a free form where anyone can reply to anyone's tweet. While in a classroom everyone would have to wait their turns and pay attention to each other. Sometimes I forget what I want to say when its my turn. So for twitter since its text based, that is less likely to happen. What I didn't like was the character limit since every tweet I wrote was too long so I had to constantly rewrite them to make them shorter while maintaining a cohorent message.
Blackboard Discussion isn't that much different than Twitter, except its more organized and takes longer. So in terms of following a discussion, it is much more convinient using blackboard, but in terms of speed it might be annoying for impatient people. A distinct advantage is the lack of character limits. And if there is, its much less strict than twitter.
Overall I would prefer Blackboard discussion over the two, and in general I don't like using either.