Verbs simple aspect Continuous(progressive) aspect Perfect and Perfect Continuous aspect

The number of students will fall over the next five years.

The number of students is going to fall over the next five years.

Sentence a) is a statement of future fact or of the speaker's beliefs.
Sentence b) is also a statement of future fact but it emphasizes that the speaker is thinking of present trends. It makes the speaker seem more involved and is thus more informal.
Expression of willingness
Will is often used to express willingness.
Be going to does not have this meaning:
I’ll help you as much as I can.
(expression of willingness: a promise)
I’m going to help you as much as I can.
(expression of intent)
Summary

• 
  Be going to always indicates a link with the present.
  

present intention/plans/preparations
present evidence of a future event

• 
  When we use will, the emphasis is more on the future than on its link with the present.
  
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