I should think that by far the best topic or topics you could use for your purpose would be the two speeches made by Brutus and Antony in Act III, Scene 2, after the assassination of Julius Caesar. Brutus prides himself on his oratory and speaks in a formal, logical manner. Antony tells the assembled mob, "I am no orator as Brutus is, But, as you know me all, a plain blunt man . . ." Antony's spontaneous, unrehearsed funeral oration, possibly the best thing Shakespeare ever wrote, turns the sympathies of the mob completely around and forces the assassins to flee from Rome. You could describe the two speeches separately and then compare and contrast them, possibly explaining why Antony's was so much more successful in achieving its purpose.