I too live in Dartford and my son attends DGS. I love the fact he can walk to school, he does not have to get up at the crack of dawn and has started and probably finished his homework by the time other boys that live a fair distance away, have got home.
The boys had an away rugby match this week, with a meet time of 7.30 on a Saturday morning......fine when you live locally, bleary eyed if you don't.
DGS is a fab school with wonderful opportunities.
I have experience of StO and TBH there is not a great deal of difference, just personal taste. My view when choosing for my DS, was, why go miles when there is the equivalent down the road!