I have tried both on a monthly subscription and my opinion as follows:
1: Napster has a slightly better library.
2: Spotify Premium sounded better than Napster to me, Napster
also buffers tracks when you skip, so it is a lot slower than Spotify for
skipping tracks.
3: Napster has better library management than Spotify .
4: Napster is better value for money, as the monthly fee includes downloads.
5: Spotify premium has a mobile App, it also allows offline playback.
Cheers Craig