There was some dialogue in TNG's "The Best Of Both Worlds" about the main deflector dish being the best designed place on the ship to channel that much power at controlled frequencies. Since phasers are generally recognized to be able to deliver damage ranging from stunning a population on a planet's surface (in TOS) to wholesale disintegration, that dialogue would suggest that the main deflector systems are more powerful than phasers. In the episode above, they had to evacuate the lower few decks of the saucer and the forward half of the stardrive section in order to "fire" the deflector dish against the Borg cube. And IIRC, they burned it out (at least for the purpose of using it as a weapon) when they fired it.
Could you use the nav deflector as a weapon? Yes. But it seems to be more of a one-off.