Brutus is the better leader of the conspirators. In more than one occasion Cassius would have cracked and Brutus kept it together. Brutus also had a better voice in the community. Brutus was able to rally a lot more positive acknowledgement of their deed, and held more authority amongst the conspirators. Brutus was a stoic that could keep his feelings and thoughts to himself and not sketch out if something appeared to be amiss. .
When the deed was about to go down, a stranger told Cassius that he hoped things go well for him and his friends. Cassius almost lost it, but Brutus stepped in to take charge and told Cassius to be constant. Cassius also said, that at the point when Caesar told the crowd to stab him if they would, he would have done so. With out Brutus there to keep him in line Cassius could have easily ruined the whole thing by trying to do it boldly and publicly out of rage. Chances are he would have been stopped and arrested or killed. If it weren't for Brutus the conspirators would have gone to get Cicero and chances are Cicero would not have agreed and possibly sided with Caesar and blown their cover.
After Caesar's death Brutus was able to convince the crowd that they"re deed was noble, well planned and well thought out. Cassius would, by his nature, get very angry with the crowd for booing him onto the stage and flipped. Cassius would have been angry and emotional, but not logical. He would not have been genuine because his reason for killing Caesar was not a noble one; his reason was because he hated Caesar. And if he had been genuine there was no ways the crowd wouldn't have tore him into tiny bits. The reason it was known that Brutus was a noble steady voice was because he let it be known and he had so many Romans that loved him dearly. When he wanted to commit suicide, his fellow soldiers loved him too much to hold his sword for him.
Brutus had more voice and more power amongst the conspirators because, having a noble drive, he made sense and could plan logically instead of being "twitchy" and random; he was collected, calm and strong.