To express a prediction, you can use either one.
-According to the weather report, it will rain tomorrow.
-According to the weather report, it is going to rain tomorrow.
- I think he will probably be late.
Use "be going to" when there is something in the present that makes the prediction certain.
Look at those cars! They're going to crash! NOT They'll crash.
To express a prior plan, use only "be going to".
-I'm going to paint my bedroom tomorrow. (I've already bought the paint.)
To express an offer to help or a decision made at the moment of speaking, just only "will".
Joe: There's going to be a lecture on speed-reading tonight.
Sue: That sounds interesting. I think I'll go.
Mary: I would go, too, if I didn't have to look after Junior.
Joe: I'll take care of him.
"Will" can also be used to make a request.
Will you please turn down the tv?