Best Answer - Chosen by Voting
Except I have heard waiters ask "do you know what you are going to have?" and the person say "I'm going to have a coffee." Now "I will have a coffee" is more common, but the meaning is the same. They are both plans. The difference lies in prior plans, and in willingness.
To clarify further:
Be going to (but not will) is used to express a prior plan (i.e. a plan made before the moment of speaking).
Will (but not be going to) is used to express willingness.
NOW, IN THIS Case, Will expresses a decision the speaker makes at the moment of speaking.
"The phone's ringing."
"I'll get it."
