There are a few differences, of which at least one overlaps. The sharpest difference however is the difference between making a decision and having already made a decision. At the very moment of the decision you need to use "will," and after that decision has been made, use "going to."
For example: I hear in the office that Chuck needs a ride home. Because I'm such a nice guy I say, "Hey dude, I'll give you a ride!" [time of decision] Later, another coworker asks if Chuck found a ride and I say, "Yeah, I'm going to give him a ride" [after the decision was made].
To the best of my knowledge, you can use either one when making predictions.
E.g., "The San Diego Padres are going to win the World Series" or, "The San Diego Padres will win the world series."
Some may argue however that there's a nuance of difference here in terms of how sure the speaker is of their prediction ...