@Bella369 I would take very little notice of the Sunday Times list in terms of rankings. They rank schools on A-B at A Level which is no great measure for private schools (ie most kids at most top 100 private schools should be getting a B at A Level). If you want to dig more into academics, look at results pages of the school websites, and the % of grades 9-8 at GCSE (ie A), and the % of A*-A at A Level. They're much more revealing as to eventual attainment (though, as discussed on here at length, you can never aim off for the raw intelligence of the 11+ cohort vs the actual quality of the teaching). Bear in mind that most schools lose about 10% of students at 6th form (according to the reviews in the Good Schools Guide).
With new heads at both schools, it has to come down to your gut feel. Where do you / DD think she'd be happiest? Which school had the kinds of girls your DD wants to be friends with / aspires to be like? Which school excels in the subjects and extra-curricular activities your DD is passionate about?