Okay this was a tough one because on TRAITS she's a Hufflepuff. Really she has all of them down. Dedication? Perhaps way too much. Hard work? Again too much. Loyalty? Yep. Patience if she's on a job. What's more is that she values all these things too. But she's not driven by them which is the main thing. And Cara is a very driven person.
If we go by what drives her...ambition. She's as ambitious as any Slytherin and makes no secret about it. She is resourceful, and has a degree of self preservation, but she's not someone who looks to become a leader. Still, Cara is someone who feels like she always needs to be moving, to be making progress, she hates being stagnant. She's goal-driven. She is going to be the best she can be and she has the ambition for that. Slytherins are destined for greatness because of their ambition and Cara falls right in that.
She'd be that Slytherin that has friends in other houses, hangs with those snakes who are decent people, and honestly would rub the purist types the wrong way. But she'd be that uber determined type who would join the quidditch team and train so hard and be so loyal to it that really the other Slytherins would have to like her for that. While in the book the Slytherins are purist traditionalists that crave power, if we go off what just what Slytherin traits are (cunning, ambitious, resourceful), none of those things are actually bad on their own. She'd like everyone in Hufflepuff but Slytherin would be better for her, she shines more when she has competition to drive her, and Hufflepuff wouldn't provide that.
Those would be her top 2 by far. Ravenclaw she'd do well enough, and she thinks it's important to keep your mind sharp, she values intelligence, but honestly she doesn't learn for learning's sake. She learns for self-improvement which is a far more Slytherin thing, it's a self-serving and goal-oriented thing. For Gryffindor, while she is daring enough, she's not reckless, chivalry isn't her thing and she's not the person who does adventure for adventure's sake.