Emusic does not include much commercial pop at all, and is a bad choice for someone who wants that.
Napster has a new service called “Napster to go” which allows you to pay $15 a month for unlimited downloads. The catch is that all the music goes away when you quit the service (it uses DRMed files, so don’t use it if your player can’t play .wma files…). It’s more of a “music rental” service, and it looks like it has a lot of the current pop.