I think Michael Corleone was the more ruthless of the two, shown when he killed his own brother where as Montana didn't kill the family in the car.

I preferred Tony Montana at first for his blunt approach but having seen Godfather more than once I've begun to like Michael Corleone more for his cunning approach.

I was surprised to see that Al Pacino didn't get Oscars for, arguably, three (GF1,2 and Scarface) of the greatest films ever made. He was unbelievable in the films and there isn't anyone who could have matched his performances.