I am convinced Oli and TB are right on this one, which would mean that Sonny and Michael were right as well. Given that when you watch the scene play out, it is Sonny who first tells Tom he wants Sol dead, and it is Tom who taks him out of it, in order to "patch things up." It is only when Tom prevails on Sonny and Sonny says, "I'll wait," does Michael speak up. His approach is very cold blooded, and very well reasoned. He gets Tom on board by selling him the idea that they can get newspaper people on the payroll to smear McCluskey and minimize the murder of a police captain. Then after Sonny mocks him, Michael convinves him be coldly telling him "Its not personal. Its business." It is at that moment that Michael becomes the de facto head of the family.