Seth Godin has an interesting take on a dozen or so forms of communication, which he's arranged on two axes. "On the horizontal, they rank from asynchronous (meaning the creator and the responder are separated in time--like a letter) and synchronous (meaning the creator and the responder are in real time proximity to each other--like a phone call)."
There could easily be a third dimension to be considered: static vs. dynamic content. Think about it, upper right quadrant communications (synchronous-highbandwidth, in Seth-speak) implies content where only the opening line is known, everything else builds on the dynamic of a conversation. Here, the content is less controlled, which can be a good thing depending on your desired outcome of the communication.
