Cassius to Brutus
Though they both conspire to kill Caesar, Cassius is more likely to lean on treachery and tricks and to play on ambition than Brutus, who is guided by his loyalty to the state. Remember that Cassius is the one who organizes the plot against Caesar, and he's running around half-naked through the streets in delight over the whole affair. Brutus, by contrast, needs to be persuaded and hesitates to join the plot without careful consideration. He reaches his decision to join the conspiracy based on rational deliberations, and he isn't overjoyed with the plot like Cassius, possibly because he has none of Cassius's jealousy or hatred of Caesar. Cassius is shady and even goes so far as to write fake letters to persuade Brutus, while Brutus relies on his own reason and his noble obligations as a Roman to do what must be done.
Brutus to Antony
The traits of deception, ambition, and treachery that define Antony contrast with Brutus's honesty and naiveté. Another contrast between them is the simplicity of their intentions when they speak. Antony often speaks loftily and with passion, as when he weeps over Caesar's body. Brutus's feelings are no less strong, but he keeps them inside. When Brutus speaks, it's without flourish or rhetoric, and he expects people to be moved by his honesty and rational thinking. He fails to realize that he can't hold others to the same standards to which he holds himself.