I disagree slightly with summerends. Westminster's girls may all be new but a good proportion, possible the majority, will have come from independent London days schools. If they dont know each other already they will only be a Facebook friend or two away. Boarders are a minority out of a fairly small intake of girls which will be fine if you form a friendship group with other boarders, boys and girls, but isolating if not. (Its really the point about whether she really wants single sex or co-ed.) Otherwise it could be isolating if your friends are day pupils with long commutes.
That said none of these schools will have offered her a place if they did not think she would integrate into the wider life of the school. If she is musical Westminster has fabulous opportunities, though competitive girls sports are less well catered for. WA will offer a broader boarding experience.
The various girls we have known who went to WA were all extremely bright, and on reflection all from international backgrounds. WA will do both humanities and science well, and a level of academic diversity amongst fellow students is a good thing.
She has done really well and both schools are more than capable of taking her through to the next stage. The choice should be based on where she feels most comfortable, where she will settle fastest, and where she feels she will have the richer wider school life. This is a crucial two years when children mature into adults. Results should be a given. She also wants to achieve independence, maturity and self confidence.