both are important. what is more important however, is the breed's potential for inflicted damage. in this regard, pitbulls are at the top. to believe that all dogs do the same amount of damage can be dangerously naive
The premise is false. What isn't dangerous is the breed of dog. What is is the lack of training. Even a springer spaniel can be dangerous if improperly trained, but you'd never see someone try to force a license for those.
the most dangerous breeds, when judged by their potential for damage and genetic disposition for aggressiveness, are pitbulls. All dogs bite, but pitbulls do the most damage, and are almost always responsible when a human has been killed.