It’s not easy to discern which to choose among ‘be going to’ and ‘will’ to express a near future plan or intention. I’m going to bring up two cases.
[A] In a situation that when one says “Oh, I’m hot,” while two are in the room, what can the other one say?: [A-1] I’m going to open the window. [A-2] I’ll open the window.
For ‘be going’ is concerned with ‘present intention’(CGEL,p.211-2), [A-1] seems to be possible; yet it’s also concerned with ‘present arrangement,’ it doesn’t seem to. How could I have arranged the opening window before his saying 'hot'?
However, [A-2] seems to be quite wilful to say in the situation. When it’s not a big deal to open the window, do I have to use my will? It could be said that 'will' is used in a implication that 'if you want, I will open the window'- as a conditional consequence.
How about this?:
[B] In a situation that I’ve planned that I would learn to play the guitar after deeply impressed by a guitarist, what can I say?: [B-1] I’m going to learn to play the guitar. [B-2] I will learn to play the guitar.
Best Answer
At the point of saying this, you already made the decision to open the window, since in effect you are agreeing it is too hot and you are offering to (want to) do it.
Here you have the intention to learn to play the guitar, but it is a long process and it is uncertain whether you will be successful or not. So this is a "present intention".