Both characters possess positive and negative qualities, which is why one could make a moving argument for either. Despite his obvious flaws, Cassius is depicted as a better leader than Brutus because he is less likely to make significant errors and is a shrewd, manipulative politician. Although Cassius is a selfish, jealous man, he possesses a clear understanding of what motivates individuals. Cassius successfully manipulates Brutus into joining the conspirators and executes his plan by influencing the disgruntled senators to assassinate Julius Caesar. He brilliantly garners support for a dangerous plan and gets his way.
Cassius also demonstrates his understanding of people by suggesting that the senators kill Antony and advising Brutus to not allow Antony to speak at Caesar's funeral. However, Brutus makes the terrible decision to allow Antony to live and gives him permission to address the masses, allowing Antony the opportunity to incite a riot and turn the crowd against the senators. Cassius is also a better military leader than Brutus and advises him to not march towards Philippi to meet Octavius and Antony's troops. Unfortunately, Brutus makes another costly decision by dismissing Cassius's suggestion, which results in a definitive loss at the Battle of Philippi.
Overall, Cassius's ability to read others, his discerning nature, and inherent instincts are what make him a better leader than Brutus in Shakespeare's Julius Caesar.