The meaning of the two idioms is not exactly the same, but in the specific context of your example, the overall meaning of what is being said is very similar.
To 'take someone for' means to accept them at 'face value', or exactly as they appear. Using this idiom in your example could suggest the girl projects a false image of herself but has another concealed personality.
The original version of your example suggests that the person has not seen a particular side of the girl's personality. There is less of an inference that she has deliberately tried to portray herself differently, although it could be said that all people act differently in different contexts to a degree and that those we interact with in those contexts have only seen part of our personality.