We were in the same position last year, my daughter passed the Wallington exam and then sat the exams for CH and OP. She's now happily settled at OP and has made lots of friends from a variety of backgrounds. Some have come up through OP's junior department, others from state primaries and private preps, but they all seem to mix very happily together and there's a great emphasis on being supportive of one another.
CH and OP are both excellent schools with strong academics and a wide range of extra-curricular activities. If you're looking for somewhere sporty, I think CH may be a better bet, but we've been very impressed by the dance provision at OP.
We were impressed by the extent to which the Head of OP took a personal interest in our daughter when she went for her interview. She had to take something with her which was special to her, and they spent a long time talking about her interests, as well as some reasoning questions. She was then offered an all-round scholarship and an additional award for Maths.
The CH interview was very different (a group interview involving reasoning tests and questions about a text). Our daughter didn't meet the Head and had no opportunity to talk about any of her interests. She's quite shy in new situations until she feels settled, and we felt that the pastoral care at OP would be a better fit for her. On the basis of our experience of the interview process, we were concerned that she would be just another face in the crowd at CH. Our impression was that CH would suit more outgoing and confident girls, but I hope someone with a daughter at CH will be able to give you the view from the inside.
What did your daughter think when you looked around the schools? Did she have any preference?
Good luck with your choice