I take it you haven't got a younger son who will need a secondary place? That's what usually swings it for Aske's.
Prendergast is much smaller than Aske's and of course it's girls only except for the sixth form. My daughter went there from year 7 to year 11. I have a lot of confidence in the head, the executive head and the governing body. It was not a perfect school but it was very good. One of my daughter's year is at Oxford now, and they have a steady stream of girls going on to good universities to do well-regarded courses. However, it's been three years since my daughter left and of course now they are managing two other schools as well. I don't know how that's affected things. They are in the middle of a rebuild of the lower site which should improve their facilities a lot.
Aske's, of course, is also the senior partner in a growing federation. Again, I don't know how that affects things.
I think if I were in your position I'd be weighing up the size of the school, ease of home to school journey, whether you and your daughter prefer mixed or single sex, any differences in curriculum, where her friends are going - all the obvious things. What does your daughter want to do?
If you possibly can, I'd visit both with your daughter and see what you feel afterwards. Good luck! It's a very strong position to be in, as you know.