First of all, I am 'assuming' you're using an iPhone. Anyway, if you select the dictation option within the Messages app and then send a voice message to a recipient, he or she will hear exactly what was spoken by you. If you tell Siri to compose a message for you and then proceed to speak what you'd like the message to state, in my experience, what is spoken is exactly what is printed in the message. Depending on the clarity in which you speak, the results should be the same. Anyway, test it out and see for yourself...