It would depend on the dialogue. Some dialogues are essentially Plato's treatises written in dialogue format. Other dialogues, like the Symposium, specifically claim to be paraphrased retellings of the words spoken by individuals at a party.

Obviously these dialogues pertain to arguing about specific topics, not everyday speech as such. So if anything they would portray how the most educated members of the society really argued with each other.

Ironically enough Socrates was very critical of writing. In the Phaedrus he says:

I cannot help feeling, Phaedrus, that writing is unfortunately like painting; for the creations of the painter have the attitude of life, and yet if you ask them a question they preserve a solemn silence. And the same may be said of speeches. You would imagine that they had intelligence, but if you want to know anything and put a question to one of them, the speaker always gives one unvarying answer. And when they have been once written down they are tumbled about anywhere among those who may or may not understand them, and know not to whom they should reply, to whom not: and, if they are maltreated or abused, they have no parent to protect them; and they cannot protect or defend themselves.

Socrates goes on a long tirade about the disadvantages of writing (written down by Plato of course) in this dialogue but stays mostly around this point that writing, unlike speech, is inherently rigid and lacks the dynamic aspects of speech that allows a debater to engage with his opponent.

So it seems reasonable to assume that Plato's dialogues accurately reflect the argumentative methods employed by Socrates and presumably others during debates/arguments. But just as a written speech doesn't portray all the nuances of the speech's actual delivery, written dialogue would be a solidified and rigid representation of a fluid, spoken argument.

Any answer to your question is going to be based on assumption since the only records we have of verbal speech are written.