We can use the present continuous to talk about the future when we have already arranged to do something. It's definite, not just an idea. Perhaps we have put the arrangement in our diaries. 'Are you free at 2 o'clock on Tuesday?' 'No, I'm meeting a client.' We can use be going to to talk about something that we have decided to do in the future, but not arranged yet. It's possible that the plan might change. When I finish at college, I'm going to spend a year travelling. The uses of the present continuous and 'be going to' with future meaning are very similar and it is often possible to use either of them, with little or no change in meaning. We also use be going to for something that we expect to happen because we can see from the present situation that it is very likely. We can't use the present continuous in this way. ( http://www.bbc.co.uk/learningenglish/english/course/lower-intermediate/unit-9/tab/grammar)

Great question!!! Here's how I differentiate between them: The present progressive used for a future action implies a great deal of certainty that the action will happen. "I'm writing Tom an e-mail tomorrow" is quite definite in the speaker's attitude toward completing the task. Using the present tense sounds as though the action is already under way (at least in the speaker's mind) or is something done on a regular basis; therefore its completion is assumed. The near future (to go + infinitive) indicates an intention or hope to do the action in the future (but completion of the activity could possibly be impeded). Other native speakers of English may disagree with me.