I have two dd's - one dd is at AGS one at SGS my feelings are that they are both great schools although if I had to choose one over the other - then AGS would be my choice, it's ambience is relaxed and informal but the teaching is first class - they get the best out of my dd, her confidence and self esteem have grown over the years, the facilities are great - newish sports block, new biology and chemistry labs last year. The pastoral care is outstanding, focusing on all children settling in identifying those that needed help and aiding those - they understand that happy children and good results go hand in hand.
I know of one issue of repeated disruptive behaviour in the 4 years my dd has been there - this was dealt with swiftly - the pupil was removed from the school.
SGS although being a great school would be my second choice simply because I feel more welcome at AGS, able to speak to a member of staff if I need to. At SGS it's a little harder to speak to someone my dd enjoys it there but had a hard time settling in - the all girls scenario has its obvious pitfalls ( need I say more!). The facilities are a little older but adequate.
Sorry if I have rambled.....but the 'feel' of AGS, to me, is superb.
I gave each of my girls there own choice - knowing in the future they couldn't complain about it
Good luck with your choice!!