What must be taken into consideration is that a lot depends on the genre....As stated contemporary music is more production driven and less emphasis is placed on lyrics. Therefore it stands to reason that this type of music is more often driven by a core riff or repeating hook rather than clever or meaningful lyrics.
More traditional music has lyrics at the core.
I think whether it is lyrics or music first will very much depend on the style and genre of the song.
Interesting to read about plagiarism. I think it works both ways....I have seen many songs where lyrics are plagiarised and also a lot of songs where musical hooks, riffs, hooks chord progs and melody has been ripped off.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055