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 folks, face-to-face is preferred.

For structured discussion and exchange between web comfortable participants, on-line tools can work well, but you won't get them same level of emotional communication and flame wars are much more likely.