I went with Mitchum myself but I have a great respect both personally and acting wise for Peck.
He was known as a real nice guy and was very active in Liberal politics and Civil Rights. But he did the rebellious thing I think a little in Yellow Sky. Peck, I think was very similar to Mitchum in acting. Both had tall figures and were handsome and most importantly, they both had those great deep voices. But most importantly, they both had a low key, cool underacting style that always made them some of the coolest characters in films.
Mitchum's performance in Night of the Hunter is his best performance. Not to mention his personal favorite of his roles and movies he starred in.
I also read that when filming the climax of Cape Fear, Peck accidentaly swung too close and actually punched Mitchum square in the face. Both being professional, they continued with the scene. When Mitchum got back to his trailer, he fainted at the sheer pain of the punch, later saying, "I have no sympathy for the idiot who gets into a fight with Gregory Peck."