I'm gonna have to be in the minority who says that ultimately, neither is more important.
If you have brilliant lyrics but the music sucks (and I'm talking about real music, not rap), the song sucks.
If you have a brilliantly composed piece of music with a lyricist who's about as talented as a dead donkey, the song sucks.
The ultimate question is does the song have something important to say? If the lyrics convey a message and the musical elements support the emotional context of the message, then a lot can be forgiven. Simplistic style or bad singing voice (Bob Dylan is a fucking God and he can't sing) are easily ignored if the song pulls at the heartstrings in just the right way.
Conversely, a badly written song with music and lyrics that flow together like a pile of bricks and a block of steel, it will all be a giant bucket of shit, even if the guitar riffs sound excellent without the vocals and the singer is insanely talented.