Czemu nie 'going to'

Czesc,
Mam problem z pewnym zdaniem.
"-How long are you staying here for?
- I don't know. I haven't decided yet."
Jest to jedyna poprawna opowiedź wedlug mojej książki. Dlaczego nie mogę uzyć :
"How long are you going to stay here for".

Pr. cont. is the most usual way of expressing your personal plans./informal/
'Be going to' is for future intentions and predictions

be going to is also correct to my mind, but the meaning changes. I'II even go further, in this sentence, it's more correct and logical to use 'be going to' owing to 'I don't know, I haven't decided yet'. If you have already arranged sth, you would usually use the Present Continuous tense, here it's not been decided yet what she is going to do, that's why I'd be inclined to use the to be going to structure.

both uses overlap .