In the movie, I believe Michael's decision was personal and business. Personal maybe partly because he wanted to avenge himself on Mac, but more, I think, because he had a personal epiphany in the hospital: When he said, "I'm with you now, Pop," I (and many others here) believe he meant more than just physical proximity to his father. He was deciding to become an integral part of the family business.
I seldom disagree with you TB, but I do about the hospital scene.
Both the novel and the movie (less so than the novel, but equally so if one views the deleted scenes), make the point that Michael was estranged from his father, at least from his father's direction. When he told Vito in the hospital that he was with him, he was telling him that he, AS A SON, was with him. He was not telling Vito that he was with him as a prospective mafioso ready to take part in the family business.
