Hi! My daughter is in Year 10 at Sutton High, where she's been since year 7 (she was at a local Catholic primary school before then). Like you, we had to decide between Wimbledon High School and Sutton High School (as well as an offer from Nonsuch).
Of the three, we ended up choosing Sutton, and both we and our daughter are very happy with our decision to do so. It's a small school (600 pupils in total, for both the junior school and the senior school/6th form) and the pastoral care is excellent. (In fact, contrary to what another poster has said, Sutton is quite a bit smaller than Wimbledon which has about 900 pupils in total).
Like your daughter, our daughter is quite academic, and the school have been great about encouraging her academically (e.g. running a special programme for mathematically-inclined girls at 7.30am on a Friday morning!!) but at the same time being very aware of the pressures teenage girls are under and doing their best to counteract these pressures (e.g. she's currently part-way through a 10-week mindfulness course, and we had a parents' mindfulness evening last week, to help us too!). Both she and we are keen for her to stay on for 6th form too, and the school are running a 6th form week for Year 10 girls next term, so that they can get a feel for what to expect.
I'm sure that Wimbledon High is a great school, but when we were doing the tour and attending the exam day my heart sank when I saw how so many of the mums were done up to the 9's with a full complement of make-up and designer gear (which would put ME under pressure to do likewise!) ... thankfully it's a lot more chilled and comfortable at Sutton High.
Finally, just one word of caution: wherever you go, joining a school in Year 10 will mean that your daughter will be coming it at a stage when friendship groups have been formed (since Year 7) and it may take her a while to settle in and find a group of friends. I know Sutton High operate a "buddy" arrangement for girls who join outside the normal cycle, but that can only go so far.
Good luck with making your choice!