I think it depends more on your character than on your plot. I personally think that a diary form makes it a little more difficult- how do we know that we can trust your main character? There's also the danger of the story getting awkward and cliche if you're talking about it from a teenage girl's point of view. If you can do it well, conveying emotion without telling your reader "I was sad when he hit me. It hurt." or something like that, that is definitely a good option.
For the third person, do you mean third person omniscient? If you want to still get in her head, that could work for you. Are you planning on writing in past tense or present? All of these things will really affect the effect (ha) of your story. You want to give your reader someone they can trust and who can tell the story well while "showing instead of telling".
The fact that your story is fictional should not effect the form you decide to write it in. Instead, you should choose the form that you think will have the most impact. Is your character the type to keep a detailed diary? Or is she emotionally remote after her ordeal? Should it be told from another point of view? The gangster? Her mother? Her left shoe?
If I were you, I'd start writing the story. Try out different forms- try the diary, try third person, try first person, try past, present, future, try second person, etc. If you're not sure what form you want it in, explore the possibilities. Try something you maybe wouldn't have considered before. Write it all in text message form or something. Write it like it's her testimony in court or it's a police report (the police report one would be a good challenge- how do you convey the emotion so it doesn't get too clinical and impersonal?)
I personally would disagree with the person who answered above me. I don't like flashbacks, and I think that they are very very rarely done well. If you think that you can do it, go for it, but working a flashback so it doesn't seem like a terrible 80s movie or something is tough, especially when you're switching between tenses. When you start to go back and forth between past and present, it should be very clear that it's deliberate and you can't have any mistakes or else your narrator loses some of his/her/their/its credibility.
I would also disagree with the person below me when they say that you should forget a prospective audience. I agree that you should write for yourself, but keep in mind that the people who read the story will need to understand it as you already do. Make sure you include the details that help you make connections and understand your characters. Also consider what they will think about your style. Will it be difficult to understand? Will that help or hinder your story?
And as to offending people- if they're offended, they shouldn't read it. Write what you want.
u never knw wat may or may nt take the wrld by storm