I don't think there's any difference between song lyrics and poetry besides the fact that one is written to be sung. I write poetry for poetry and songs for songs, so I do make a distinction. You can't expect to write a free-verse poem, and then be able to sing it over any old melody, but I think it works the other way around. If your song is beautiful and inspiring enough, why not consider reciting it instead of singing it? If you're into that stuff, that is.
Personally, I put a lot of thought into my poems. When I decide to write a poem, which is rarely, I put my heart and soul into it. I don't just write, I think. I try to use a lot of literary devices, and take time carefully choosing the right words. I try to focus more on the feelings evoked by the words rather than meter or rhyme. I want my poem to really grab a person when they read it, and leave a lasting impact.
With songs I'm less picky usually. There's been a few songs I've written where I've started with the lyrics. These songs are usually songs with a lot of meaning for me, usually a social commentary, a personal memory or reflection, an idea, or something I've noticed about the world. Most of the time, however, I start with a melody, less commonly with the entire accompaniment finished without even thinking of the lyrical theme! I've thrown together lyrics in about a minute before and kept them just because I've like the spontaneity of it. On average I'd say it takes me about 45 minutes to an hour to write the lyrics for a song. But like I said, I'm not picky. For songs, it's the music, the melody that matters, not the words for me, at least regarding my own music at this point.
