Well now the cat is out of the bag so too speak, media will be camped outside the DW studios to try and catch a glimpse on the next Doctor.
If the role is cast, then, yes, the BBC is best served by announcing it sooner rather than later. Stay in front of the story, don't let the media ferret it out, and don't have a panic reaction like Saturday's announcement that Matt Smith is leaving.
I don't know if Moffat would try to hold the next Doctor announcement until SDCC or not. I can see arguments both for and against.