How is Mark Antony more astute than Brutus?

Both Antony and Brutus are intelligent, but they have different temperaments. Brutus is an introvert and Antony is very obviously an extrovert. Brutus is unworldly, while Antony is very worldly. Antony is always characterized as a man who loves having fun. He is very self-indulgent. He likes sports. When he first appears with his good friend Caesar he is prepared to run in a foot race as part of the celebration of the Lupercal. Since Antony devotes so much of his free time to sensualism, it is clear that he cannot have very much time to devote to study. Brutus is just the opposite. He is shown reading a book even on the eve of the battle at Philippi. This is a real bookworm! Antony is able to take advantage of Brutus because he is worldly wise. He might be called "street smart," to use a modern term. Brutus understands books, but Antony understands people. He knows how to get around Brutus even when he is in the deepest trouble of his life. He would have been killed if Cassius had had his way.

Cassius is another worldly wise man like Antony. Cassius can read Antony's mind. He tells Brutus earlier that Antony might prove to be "a shrewd contriver." Caesar is just as worldly wise at Antony and Cassius. Caesar says of Cassius:

He reads much,
He is a great observer, and he looks
Quite through the deeds of men. He loves no plays,
As thou dost, Antony; he hears no music;
Seldom he smiles, and smiles in such a sort
As if he mock'd himself, and scorn'd his spirit
That could be moved to smile at any thing.

Antony does prove to be a shrewd contriver. He knows just how to manipulate Brutus and manages, not only to save his own skin, but to get permission to speak at Caesar's funeral. He tells Brutus he is willing to join the conspirators in their proposed new government if Brutus can explain his reasons for killing Caesar. Brutus jumps at the bait. He loves talking about reasons, logic, abstractions. When he speaks to the citizens he emphasizes his reasons for leading the conspiracy against Caesar. He really has no concrete evidence, only his suspicions of what Caesar might have done if he had been allowed to life.

Antony doesn't really care about reasons. He is a pragmatist. He cares about whatever works. But he conveys the impression that he is a fair and reasonable man just like Brutus. Brutus makes the mistake of trusting Antony. Unfortunately, as Shakespeare shows, it is often a mistake to trust people in our lives. Brutus' willingness to trust in the good will of others leads to his downfall. Antony does not even seem embarrassed about double-crossing the more noble and idealistic gentleman. Antony comes out on top because he is a realist and a pragmatist, cynical and unscrupulous. In Antony and Cleopatra, Shakespeare shows how Antony is destroyed by Octavius, who is even more of a realist and pragmatist, more cynical and more unscrupulous. 

Cassius also understands Brutus and knows how to manipulate him. After they separate in Act 1, Scene 2, Cassius says to himself:

Well, Brutus, thou art noble; yet, I see
Thy honorable metal may be wrought
From that it is disposed; therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced?