Hmmm.
GDST are normally all girls, but day rather than boarding (think South Hampstead High, Notting Hill and Ealing High, Putney High, Wimbledon High, and and and.) It is the Girls Day School Trust, and tend to have slightly lower fees and a more down to earth nature than some of their more swanky competitors. I hope I dont get flamed for a huge generalisation!
St Mary's Ascot, like Beneden, is very much a girls boarding school.
I think that it is important to look at the child first and then the school, rather than simply at a league table. I have at least a couple of perfectly intelligent friends who, because of having gone to schools filled with the brightest of the bright, have never really believed they were bright enough. (Even a subsequent degree from Cambridge was not enough for one.) In our case it has taken DD a while to fully enjoy and engage in education and I am not sure if she would have got there had she been in a more hot house environment where this engagement had been assumed from the start and where she might have started towards the bottom of her age group. Quite a pushy prep had left convinced she was not academic, and it has been fun to see her gain in confidence since then.
The girls we know who went to WA were very bright, perfectly nice, quite self contained and focussed. Girls who would thrive anywhere. I dont know enough about the school to know whether it would suit the less confident or less accomplished, but I think it is fair to say that different schools have different cultures, and different cultures suit different girls.