Both, pretty much at the same time. Sometimes I'll be fooling around on guitar and hit upon an interesting riff that inspires a lyrical idea. Other times, a line or two will pop into my head (almost invariably when I don't have my guitar handy!) and I'll noodle around until I find a guitar part that fits it. But either way, once I start working on a song, the music and lyrics usually evolve together. I'll usually finish the lyrics before I finish the music, because it's easier to tweak lyrics in my head wherever I happen to be than it is to drag my guitar along to work or to the grocery store.
Then again, sometimes I just start with the title.