There is a clear difference between them. We use going to when there is smth in the present situation that shows what will happen in the future. The speaker either has made some plans or is sure about what will happen.
I'm going to visit Mr.Pedantic at the weekend.
Look at those black clouds -it is going to rain soon.
The Israelis are going to continue punishing Lebanon for a week or two.
We use the Future Simple if
a) the decision has just been made
b) we are not absolutely sure it will happen; there are some conditions
-Oh, we,ve run out of sugar. - Have we? I'll go and buy some.
The bombardment will stop if Americans pressurize Israel.
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