Overall PvP is much more enjoyable then PvM because of the fact your fighting other people, who are living and actually can use tactics against you rather then having a Monster/MOB use one or two straight attacks.
I like games for their multiplayer features, for example; I brought black ops on it's relase date and I didn't bother doing the campaign for over 6 weeks.
PVP is better because when you vs monsters, the majority of them cant eat and cant switch to a spec or doesn't have stats like you do. In PVP the player is just like you, can switch, chooses their stats, and even has different items the different people you verse.