Tom Brady may have the Super Bowl rings and the supermodel wife, but here's one way Peyton Manning appears to have him beat: marketability.

The Nielsen Co. partnered with E-Poll Market Research to create the "N-Score," a statistic that measures the endorsement potential of athletes. To determine the scores, Nielsen administered surveys to 1,100 Americans above the age of 13. The results are based on the public's awareness of the athletes and on a series of questions pertaining to their appeal and attributes.

The...