a) Predictions expressed by will are based on your intuition and experience:
b) Predictions expressed by going to are based on what's happening at the present moment:
Going to usually refers to the near future. While will doesn't refer to any specific moment (it may sometimes concern very far future):
This is a subtle difference but it is noticable: if someone uses the going to form, the person is often talking about his or her intentions.
In other words, there is an emphasis on the intention of doing something: