I've had both running for a little while.
Spotify sounds better, plays tracks quicker after selection and of course has the mobile app.
Napster has more tracks (Wonderwall for example) than Spotify, has an easier menu system for new releases and can be used through any web browser - but no app. They've also had terrible problems with their Sonos service.
Spotify costs more, but for me is worth the additional money. I would like to see an increase in the SQ on the mobile app though. So I'll be cancelling Napster.