Michael was every bit as intelligent as his father, perhaps even more so. If he were not, he wouldn't have expanded the Family as he did.

What his did not have was his father's 'finesse'...Vito's intuitive gift of understanding and knowing how to deal with people on a personal level.

And once Sonny died, he really did not have 'a choice' to leave. To do so would've been to abandon his father, which he would never do.

So technically he had a choice, but in reality he didn't.

Apple