Actually 'going to ' is more concerned with intentions . I am going to have lunch at 3 o'clock! We are going to buy a new house next month ! Whereas ' will' is stating facts about the future . The sun will rise tomorrow at x o'clock. The shops will close later during the vacations.

Generally, "will" is the auxiliary verb used to express the future, and I think (at least nothing to the contrary is coming to my mind), you can use that and not be wrong. It's particularly to be preferred if there is indeed a notion of control over the situation or of "having a will", meaning voluntarily wanting to do something. "Going to" is sometimes a better alternative, and you'd use it when someone has a plan to do something, or if something is happening on a schedule. For example "It's going to rain tomorrow, so I will take my umbrella". There is a less of a notion of doing something just because you want to and more because events are carrying you along. So, for example. "I want to do something nice for Sue's birthday, so I think I will bake her a cake", but "I have to bring something to the potluck, so I'm going to bake a cake". But again, note you could use "will" in all my examples and be OK.

Thanks hmano :)

Thanks Reynolds :)