Whereas the Reverend Mr. Dimmesdale is the inadvertent messenger of secret sin with his congregation, the Reverend Mr. Hooper is an intended one.  In Chapter XI of The Scarlet Lettter, Hawthorne writes of Dimmesdale,

While thus suffering under bodily disease, and gnawed and tortured by some black trouble of the soul, and given over to the machinations of his deadliest enemy, the Reverend Mr. Dimmesdale had achieved a brilliant popularity in his sacred office. He won it, indeed, in great part, by his sorrows.

Thus, without his intentions and against his humble protestations--

"I your pastor, whom you so reverence and trust, am utterly a pollution and a lie"-- 

Dimmesdale is perceived an inspiring minister and even a saint.

On the other hand, the Reverend Mr. Hooper intentionally sets out to awaken his congregation to the secrets of their hearts that they veil in sanctimony. For, he dons the black veil in order to model to them their own hidden weaknesses that they should reveal to themselves as part of the human condition. This he verbalizes on his deathbed:

When the friend shows his inmost heart to his friend; the lover to his best-beloved; when a man does not vainly shrink from the eye of his Creator, loathsomely treasuring up the secret of his sin; then deem me a monster, for the symbol beneath which I have lived, and die! I look around me, and, lo! on every visage a Black Veil!'’’

Interestingly, as the unintentional messenger, Dimmesdale is much more popular than Mr. Hooper, from whom many flee lest they perceive too much of themselves. For, it is only after Dimmesdale's confession on the scaffold that the Puritans realize his human weaknesses and are, thereby, awakened to the realization that in Dimmesdale's sufferingthey have earlier recognized their own sins, while Mr. Hooper awakens his congregation as soon as he dons his veil.