Це та сама ймовірнісна характеристика настання події (відбудеться/не відбудеться).
Знайшов ось таке пояснення:
"I will ~" and "I am going to go ~" mean that I do not doubt that it will happen; it is going to happen; I see no reason why it won't happen.
"I plan to ~" somewhat implies that maybe it won't happen.
Usually, if you are speaking of something far in the future, you use "plan to".
Next summer, I plan to go to Germany. [But, I might not be able to.]
This weekend, I'm going to complete my project. [and I'm sure of that.]