As a practitioner I find that face-to-face dialogue provides for greater trust and emotional communication based on eye contact and body language. As well, many people are not comfortable with text chat and writing and thus are disadvantaged by on-line and written forms of dialogue. Also face-to-face can be much faster for exchanging perspectives and opinions. Thus for dialogue aimed at finding common ground between average folks, face-to-face is preferred. That said, outcomes from a face-to-face discussion should be documented on-line.
For discussion and exchange between web comfortable participants, on-line tools can work well, but you won't get the same level of emotional communication and flame wars (e.g. insults and personal conflicts) are more likely.