Brutus would be a better ruler than Cassius, but Cassius might be a better leader in the preliminary stages leading up to establishing a new government. Brutus is honest, unselfish, just, compassionate, judicious, and charismatic. Cassius is in many ways the opposite of Brutus, but he is cunning, and worldly wise. Julius Caesar says of him:
He reads much.
He is a great observer, and he looks
Quite through the deeds of men.
It was Cassius who initiated the assassination plot. Without him the others would never have acted as they did. Yet Cassius knew he was not well liked and that he needed Brutus to give the conspiracy the justification it needed. Shakespeare gives an illustration of the general feeling towards Cassius when he asks Casca, "Will you dine with me tomorrow?" and Casca, who serves primarily to represent all the other conspirators in the play, replies:
Ay, if I be alive, and your mind hold, and your dinner worth the eating.
Casca doesn't like Cassius. He knows him to be a selfish miser. He is being deliberately rude. He knows Cassius would never offer him a dinner if he didn't want something in exchange, and he knows what kind of dinner to expect. Casca would never talk to Brutus in that tone because he has too much respect for him. Casca's attitude shows why Cassius needs someone with the prestige of Brutus to act as leader of the conspiracy he alone is fomenting. But once Brutus is committed he becomes impossible to control. He contradicts Cassius in everything.
Cassius knows Antony is dangerous and wants to have him killed. Antony is cunning and deceitful, and Cassius knows he is not to be trusted because he judges him by himself. But the noble Brutus overrules Cassius. Then when Brutus grants Antony's request to speak at Caesar's funeral, Cassius says:
You know not what you do. Do not consent
That Antony speak in his funeral.
Know you how much the people may be moved
By that which he will utter.
But Brutus overrules Cassius again. Brutus is infatuated with his own nobility, his distinguished family, his prowess as an orator, and his superior wisdom. Brutus makes many serious mistakes, but Cassius made a serious mistake in persuading Brutus to become the leader.
Cassius does not want to fight Antony and Octavius at Philippi, but Brutus overrules him again. If they had somehow managed to win that battle and had established a new government in Rome, Brutus would have probably been a wise governor; but Cassius would have been a better leader of the original conspiracy. What he was obviously hoping was that he could be the de facto leader and use Brutus as a figurehead. They would have made an excellent team if Brutus had not been so vainglorious and headstrong.
The partnership was doomed--as so many partnerships, including marital partnerships, are. Brutus despises Cassius for being so dishonest and materialistic. Cassius does not really admire Brutus's integrity; he probably sees him as a pseudo-intellectual and a fool. Cassius shows his feelings in the big argument in the tent when he says:
Brutus, this sober form of yours hides wrongs
And Brutus shows his feelings when he says:
Let me tell you, Cassius, you yourself
Are much condemned to have an itching palm,
To sell and mart your offices for gold
These two would not remain partners for long. If they had become co-rulers of Rome, Cassius might have ended up assassinating Brutus, since Brutus is too noble to think of assassinating Cassius first.
Brutus is the better leader because he gets persuaded into the assassination by Cassius and he is the only person among the conspirators that believes that once Caesar is killed it will benefit Rome. Everyone else is killing Caesar out of blood-lust, envy, or rivalry.