I do both separately. I have been writing for years, before I had a guitar, so I don't need music to write good lyrics. Sometimes when I'm just messing around on my guitar, I'll find a cool chord progresson and I'll write it down and save it for later. If I had to choose, I'd say that I write the lyrics first. I might have a cool riff or something that would sound cool and I just build off it when I'm writing the song. I guess what I'm saying is that the music is more dependant on the lyrics than the lyrics on the music......at least for me.