You can also take a look here:
http://www.sk.com.br/forum/display_message.php?message_id=17352
http://www.sk.com.br/sk-perg3.html#109b
WILL x BE GOING TO:
1) To express a prediction, either one can be used. (Para prever algo que venha a acontecer no futuro, as duas formas podem ser usadas.)
Ex: According to the weather report, it will rain tomorrow. / According to the weather report, it's going to rain tomorrow.
Be careful! You'll hurt yourself! / Be careful! You're going to hurt yourself!
2) To express a prior plan, only BE GOING TO should be used. (Para nos referirmos a uma ação futura, porém como resultado de um plano preestabelecido.)
Ex: Why did you buy this paint? I'm going to paint my bedroom tomorrow.
Are you busy this evening? Yes. I'm going to meet John at the library. We're going to study.
3) To express willingness and intention at the moment of decision, only WILL should be used. (Para nos referirmos a uma ação futura resultante de nossa predisposição, no momento em tomamos a decisão.)
Ex: The phone's ringing. I'll get it.
If you don't understand, ask your teacher. He'll help you.
Speaking to a waiter: - I'll have a beer, please.