Is there any difference between "be going to do sth." and "will do sth."

[weiteng]

Hello folks,
For native English speakers, wherever your from, is there any difference between "be going to do something" and "will do something"?
For me, of course as a foreign tongue for the English language, when I say: "I'm going to get a Coke." That means the action of getting a Coke will happen in a short time, right after I finish this utterance.
Then if I say: "I'll get a Coke". The action of getting a Coke will happen after a certain period of time, and for how long may not be certain.
So is my understanding correct? Can anyone put in your own two cents for my question. Thanks a lot.

[Tdol]

It's often not a question of time with these forms. If I think about it first, then I'll say 'I am going to get a coke', but if I suddenly decide to do it, I will say 'I will get a coke'. 'Going to' is often used for our plans and intentions, but when we decide now, we use 'will'.