Changing Activity.start to take a c.g.web.bindery.event.shared.EventBus would be a breaking change: every Activity would have to be updated to the other EventBus.
I agree ActivityManager should take a web.bindery EventBus though.
But that's not big deal: web.bindery EventBus has mainly been added for use with RequestFactory in non-GWT apps (Android for instance), so I'd say use c.g.g.event.shared.EventBus in your GWT code and web.bindery EventBus everywhere else (i.e. where c.g.g is not available).
At least for now…