What are three contrasts between Marc Antony and Brutus?

Antony is opportunistic, ambitious and ruthless, while Brutus is optimistic, honorable and naïve.

Antony wants power.  Although he was loyal to Caesar, he took advantage of his death easily and quickly.  Brutus is not at all power-hungry.  He only cares about Rome.  He believes that he can protect Rome from Caesar by killing Caesar.

Antony is ruthless.  He is willing to kill anyone to get what he wants.  A good example of this is the proscription, where he easily condemns men to exile and death, even family members.  Antony does not think twice, because it is what needs to be done to win the war.

Brutus, on the other hand, believes in limiting violence to what they must do.  He wants Caesar’s death to be clean and seem like a tyrant-killing and not a murder.  Killing a tyrant is justified, in Brutus’s mind.  He believes they are liberators and the Romans are slaves to Caesar.

Let us be sacrificers, but not butchers, Caius.
We all stand up against the spirit of Caesar;
And in the spirit of men there is no blood:
O, that we then could come by Caesar's spirit,
And not dismember Caesar! But, alas,
Caesar must bleed for it! (Act 2, Scene 1)

Antony is not naïve.  He takes action when he needs to.  He carefully whips the crowd into a frenzy at Caesar’s funeral, and makes sure that he trades on their affection for Caesar.  Antony realized that Octavius might be a problem. He made sure he was not there when he gave the speech.  He did not want to share the credit.  Brutus had no idea that this would happen.  He made a mistake in leaving Antony alive, and a bigger one in not listening to Cassius and letting him speak at the funeral. 

Antony acknowledges that Brutus is honorable.

This was the noblest Roman of them all:
All the conspirators save only he
Did that they did in envy of great Caesar;
He only, in a general honest thought
And common good to all, made one of them.
His life was gentle, and the elements
So mix'd in him that Nature might stand up
And say to all the world 'This was a man!' (Act 5, Scene 5)

In many ways, Antony and Brutus are opposites.  Brutus is an optimist, thinking things will turn out okay.  Antony takes advantage of opportunities as soon as they arise.  For all of Brutus’s nobility, Antony is ambitious.  He hopes to gain personally from his rise to power.  Antony is also ruthless, willing to do whatever it takes while Brutus naively tries to do the right thing.