Depends on what we are listening to, really.
Things like pop, the music tend to be more like a side thing while the lyrics are important.
Things that are electronically based (I.E. Trance, Techno, etc) I'd prefer it to be mostly music.
Basically, the more sounds in the background the more I focus on the music, the less sounds in the background the more I focus on lyrics. There isn't anything that can help with bad music or bad lyrics though.
Exhibit A: Pop: Mostly lyrics.
Celine Dion - Because You Loved Me
Westlife - Flying Without Wings
Exhibit B: Electronic: Ignore the video itself, it is all follow the beat.
ABG - Recollections (Dynamis Remix)
Oliver Smith - Tomahawk
Exhibit C: Rock: Energy is from the music, power is from the lyrics.
Story Of The Year - Anthem Of Our Dying Days
Rise Against - The Good Left Undone
Exhibit D: Rap: Mostly lyrics. Music adds effect to the lyrics. Its hard for me to find rap that is alone or something that has enough rap (because most that do, I hate it.)
Linkin Park - Hit The Floor
X-Ecutioners feat. Mike Shinoda & Mr. Hahn - It´s Going Down <--- Linkin Park is pretty much in there too.
Exhibit E: Classical: All Music
Canon in D
Leaves On The Seine