If your listening is primarly desktop based, then spotify free is the way to go -- get all the access to all the music (with ads), for $0.

I pay because having mobile access is important to me (when I drive, travel, or whatever). If I wasn't using it on mobile, I would not pay for it.

And, of course, even that depends on how much music you would otherwise buy. I spend $120/year for spotify right now, which is way, way less than I used to spend on music annually.