It´s definitely not the same thing. It’s all a matter of decision. Let´s see some examples:
I´m having a party. I´m going to invite lots of people.
Here, the speaker has clearly already made a decision (before the time of speaking). The decision to invite lots of people to the party. There doesn’t seem to be any doubt about it.
Let´s see what happens here:
Hey! I think I´ll have a party! I´ll invite lots of people.
In the second example, it is clear that the speaker has made the decision spontaneously, at the time of speaking.
Let´s look at some more examples:
Sarah: “I´m going to have a party. I want to invite Tom, but I don´t have his number.”
John: “I´ll give you his number.”
Sarah has made the decision to have a party (she´s going to have one), while John spontaneously decides to give her Tom’s number (I´ll give it to you).
This brings us to another use of will. John also offers to give Sarah Tom’s number. So, will can also be used to offer to do something.
Ann: “I have a terrible headache.”
Tom: “I´ll get you some aspirin.”
What if you’re not sure about what you’ll be doing? Then, you also use will. I think I´ll see Claire tomorrow in class, but I´m not sure she’s going.
We often use I think I´ll…, or I don´t think I´ll….
What about predictions about the future? Well, you can make a prediction by using will. Marta will learn a lot in her English course in Denver. And she´ll enjoy it, too.
However, if your prediction is based on evidence, something you see now, then you should use going to. Look at those clouds! It´s going to rain any minute now.
So, here, we see there´s the matter of certainty. Whenever, you use words like probably, perhaps, I think, I believe, etc…you use will to make your prediction. If however, you’re certain your prediction will come true because all of the evidence points to it , then use going to: “I feel horrible. I`m going to be sick.”
Do you have many more doubts like these? Then consider the option of taking English courses at LCI. The teachers are experienced and able to answer all of your questions about grammar, or learning English in general.