I found an interesting article comparing Unitas to Starr by Allan Barra.
Most overrated great quarterback: Johnny Unitas. Most underrated: Bart Starr. I made this point more than a year ago, but all the sentimental postmortems of Unitas' career make it necessary to reiterate a few facts. First, while Unitas was undeniably one of the three or four or five greatest quarterbacks in NFL history, he has been vastly overrated as a big-game player. The legendary 1958 and 1959 title-game victories over the New York Giants weren't merely the most famous championship games that Unitas ever won, they were the only championship games that Unitas ever won. Johnny U's entire 1960s career was one long frustrating attempt to live up to that early big-game glory. He never came close, a fact which seems to have eluded otherwise astute NFL historians such as Paul Zimmerman and Peter King.
Second, Bart Starr dominated Unitas in head-to-head matchups during the 1960s, winning 10 of 15 straight up and consistently beating the teams Unitas couldn't (for instance, the Jim Brown-led Cleveland Browns of the mid-sixties and the '67 "Fearsome Foursome" L.A. Rams). At the time, it was argued that Starr's success was due to the marked superiority of the Packers over the Colts from 1960 to 1967, but a quick glance at the record shows us that isn't true. Over that eight-year span, the Packers won just a handful more games than the Colts. The Packers won five championships in those eight years to the Colts' none, primarily because the Packers -- primarily because of Bart Starr -- were better in big games. At one point, in '65 and '66, they beat the Colts five straight times.