I've children at both and would agree that it really does depend on your child.
Both schools get excellent results; they have very different feels, under the current regimes I'd say DCGS is actually the 'warmer' school in terms of approachability, at least in our experience. That said, the ds at CGS has had some very positive experiences there.
Think about whether a mixed-sex environment would suit your ds better; consider the logistics, particularly as county is trying to cut bus routes.
WRT sport, yes DCGS has a higher level of team simply by virtue of having double the number of boys and a higher prep school intake, however CGS can punch above its weight and if your ds enjoys it, there's every chance he'll have far more squad time.
We let our children decide their schools and they've all been happy with what they picked
