I tend to think of them basically at the same time... If I had to choose, though, I would say the tune is more important. Honestly if the lyrics can't fit a tune, then the writer has paid no attention to the rhythmic value of each word, so odds are exceptional that he's not a very good writer to begin with. If the writer can't even adhere to a certain syllabic pattern, I will guarantee he genuinely blows at writing poetry, and therefore will suck at writing lyrics first.