you dont state what you mean by best? quality or usability?
i have tried both and found spotify had a slightly better quality due to a higher bit rate.
As far as functionality goes i didnt really keep napster for too long but do use spotify a lot on my desktop, spotify integration via other devices will give you a subset of functionality and control, e.g ipad/iphone loses the whats new listings but plays playlists nicely, as has been stated you can try both for a few pounds,
I use spotify mostly to identify new music rather than playing much through sonos / hifi, i do this at my desktop while im working, fwiw i currently subscribe to spotify premium but having tried the free service i found it met my needs nicely (the advertising is not particularly intrusive and the iphone offline play lists are nice but not essential for me), for this reason i have cancelled my premium membership after 2 months.
You may want to give Wolfgangs vault a go too, free service is limited in bit rate and catalog isnt great but there are a few nuggetts on there,
hope this helps