I'm assuming that you are referring to Southend High School for Girls and Westcliff High School for Girls?? I am also going to be a bit naughty and make the assumption that you are not local to either? (Apologies if I have got this terribly wrong
). Most of us in the area have a preference for one over the other due to how we and our DD 'feel' about the school, not choosing on the basis of where it comes in a league table, I think. Both are excellent schools. If you have put SHSG first because your DD felt that this was the school that she would be happiest at, then you have probably made the right choice.
One always has to remember that results are not just the product of what the school puts in, but a combination of the school's efforts and the pupils with whom they are working and so there will be some variation from year to year anyway, even where the intake is all of GS standard.
(P.S. Our DD is in year 8 at SHSG, never wanted to go to Westcliff. So perhaps we are a bit biased here