Demosthenes versus Cicero. Plutarch knew that this was a fruitless comparison since, in his view, they were similar in "their passion for distinction and their love of liberty and civil life, and their want of courage in dangers and war...there could hardly be found two other orators, who from small and obscure beginnings, became so great and mighty."
But Plutarch writes that Demosthenes was a lousy public speaker at the start of his public life and "was derided for his strange and uncouth style, which was cumbered with long sentences and tortured with formal arguments to a most harsh and disagreeable excess."
Demonsthenes was so discouraged by the reaction to his oratory, that he gave up public speaking. While his ideas were sound, his presentation was wanting. Plutarch writes that it took the advice of the actor Satyrus to convince Demosthenes that it was "as good as nothing for a man to exercise himself in declamating, if he neglected enunciation and delivery."
So he spend a lot of time "woodshedding," in the jazz vernacular, practicing in private and working on his speaking style. He worked so hard on his delivery that his contemporaries, according to Plutarch, wrote him off as "a person of no great natural genius, but one who owed all the power and ability he had in speaking to labor and industry."
The ability to speak extemporaneously was prized by the ancients, so Demosthenes' style — apparently a blend of lots of prior preparation combined with off-the-cuff speaking — was derided by his rivals. Pytheas said his arguments "smelled of the lamp," but Demosthenes' reply was that "it is true, indeed, Pytheas, that your lamp and my lamp are not conscious of the same things."
His belief that the argument is more important than the style in which it is presented is certainly important, but, as Plutarch writes, while Demosthenes had the substance, it wasn't until he found a style he was comfortable with that he became an orator of the first rank.