Having direct experience of both schools I would doubt there'd be much to choose from the boys in the top and bottom sets of either schools ... It's perfectly possible to f*ck up your GCSEs at Dulwich as one of my friend's sons seems determined to do ...
I suspect where the academic difference might lie is in the middle tiers of both cohorts with Dulwich being slightly higher, but for any one individual the choice is going to be down to lots of different factors, like location, ease of transport - buses, trains, school bus routes (which is where you were talking tosh, CT), where friends go to etc etc
Both schools will get good/excellent results if your son's prepared to work for them (and not if they're not).
We and other families I know preferred the atmosphere at Whitgift - and I think there's a snobbery factor going on there as well - Dulwich's intake is much 'posher'. And after ds2 was horribly bullied by a load of boys from DC on a cricket course, that didn't say a lot about the behaviour of some of the boys from there ...
As far as top tier London schools go, I would agree that neither DC nor Whitgift are in it - you'd be looking at St Paul's, Westminster and KCS Wimbledon, but again they wouldn't suit every boy for lots of different reasons, not least of them being location and fees
OP - when it comes down to making a choice, if you are lucky enough to be in that position, it's really about your gut feeling. Imagine your son's first day at secondary, which school can you see him walking into? And which can you see him being happiest at?