My DD has got offers from 2 secondary private schools. Both have much going for them. The biggest difference is the single sex (girls) vs co-ed.
Our DD is an only child and, although has enjoyed co-ed education so far, most of her friends at school are girls and she does not interact much with boys in her extra-curricular activities.
Instinctively, I feel that co-ed might prepare her better socially for the world she would face at university and beyond. But would really welcome views of others with girls (either only children or in with only sisters in the household) in a similar situation that have gone to single sex schools and can comment on their experience.