(I think this is the same logic as learning the right way vs. the wrong way) However the main differences between formal and informal communication are:
Formal- Information is established from a formal channel by which sources of writting are generally taken from; policies, manuals, procedure and rule books, previous meetings, reports etc. Well rehearsed, planned and works well under a timely schedule.
Informal- Information is gathered, with valid sources, however may not be communicated as wanted and can sometimes conflict with formal communication. Usually not planned/ scripted or written out, uses a model of "going with the flow," however tends to do for engaging with the audience and viewers than formal communicators do.
demonstrates that formal methods, start off at a higher value level, but continue to depreciate as it undergoes the expert phase. HOWEVER, informal methods start off at a lower value in the beginning but gradually increases as it reaches the expert phase.