I always do the words first. But thats just me. What I do is I get the lyrics down, then I think 'what mood do I want' I put that in my riffs and solos. It depends on what you want. If you want the lyrics to stand out, write them first and use the music to set the mood. If you want something that you can just rock to, get the music down, then add lyrics. It all depends on your style and what you want people to pick up in your playing.