(Be Warned! I have a coughing fit during this.) Modern people often don't appreciate the differences found in the various gospels. We tend to think of them as accounts of what happened, but the writers thought of them more like sermons that helped their congregations - who already knew the story of Jesus - better understand the events of Jesus life, death, and resurrection. Matthew wants his Jewish congregation to identify closely with the disciples, and to hear Jesus speak to them through those disciples. And so at the Transfiguration, Matthew helps his congregation consider whose voice they listen to.
Many voices compete to be heard by us. Which do we pay attention to and follow?