napster is now a pay per download service that is legit
kazaa is a borderline illegal program (it is mostly used for illegal downloads even though the technology itself has been ruled legal) The downloads are free but the ads are an annoying part of its license. I might add you can use kazaa lite but its an illegal port of the kazaa software and you arent legally even allowed to use it.
If your comparing the old time napster to the modern day kazaa I would have to say I prefer using kazaa due to my inability to obtain a broadband connection when napster was dominant.
I hope this was close enough to the response you were looking for.