On Napster for $14.99 per month you get to mix your own music with a library of over 1 million tracks.
When you stop paying, you can't listen to your music anymore.
It allows you to listen to an incredibly wide variety of music, it's good if you like different type of music and like to explore.
Some people prefer to own their tracks, but that can get quite expensive.
On iPod u got to buy your tracks for $0.99 each and you do what you want with them. It would cost $10,000.00 to fill up a 20GB mp3 player though
I personally use Napster and love it.