Hi dazed and confused - sorry can't help as I don't know City of London school at all - but DS has been offered St O's and we are very impressed with the school and it is local to us. St O's is a maths/science specialist school but I imagine that all schools at this level will have strong focus on these subjects anyway. He will also make new friends and if his gut feeling is City - then go with it particularly if it is closer. Nice to have a choice of good schools - many people don't.
Have just realised that you will already have made your decision - sorry for not responding earlier. Good luck with whatever you have decided.