EXPRESSING FUTURE

What's difference between the use of the GOING TO Future and the Simple Future with WILL?

1-SIMPLE FUTURE WITH WILL: expresses a future action or state of being in which the element of certainty is not present:
I will travel to Paris next year. (If I have the time, the money, etc...)

2-GOING TO FUTURE: expresses a future action or state of being in which the element of certainty is present:
I'm going to travel to Europe next year. (I'm determined to go and I'm alread doing what is necessary to realize my future action).

PRESENT CONTINUOUS TENSE AND GOING TO FUTURE
They can be used to refer to a future event in which the element of certainty is present.
Paul is seeing the doctor next monday - or - Paul is going to see the doctor next monday.

SIMPLE PRESENT TENSE
It's can also be used to refer to a future event that will as part a program, or as part of as series of events that routinely take place. An adverb( like: tomorrow) or an adverbial phrase (like: next week, in three hours, ..) is usually combined with the Simple Present Tense.
Next week, he celebrate his 15th years old.