To clarify:
John Mayer is a fantastic blues guitarist, and I actually thought that his most recent album Continuum really showcased his talent.
Dave Matthews plays more of a jazz-based style of jam-rock, with catchy, melodic riffs and vocals that appeal to a wide array of listeners. His songwriting is both infectious and profound -- fearlessly commenting on love, war, life, death, fun, responsibility, and religion.
Lumping the two together, is absurd. While both are phenomenal musicians, the sound is very different.
I am actually a huge fan of Matthews and Mayer and not one bit ashamed to claim that. I've played guitar and piano for years, write continuously, listen to almost everything, and would list both of these guys in my short list of favorites.