You may consider yourself an MMOer that likes a healthy balance of PvP and PvE gameplay, but deep down, we think most people have a favorite. It's not that surprising when you look at it -- just think about how utterly different the same game can be when you are fighting NPC mobs, compared to fighting other players. Let's look at the arguments for each, starting with PvE. Some players can't get enough of the excitement of soloing an extra hard named boss, or getting together a small group of skilled friends to complete a tough instance. Others just love a good guild raid.
Over on the PvP side of the fence, you simply can't understate the thrill of the unpredictability that comes from fighting a human opponent. Does the other player have a better strategy than you? Are they just plain quicker at execution? For a lot of players, PvE is boring when compared to the highly competitive PvP world. As with PvE, this doesn't need to be a solo (1 on 1) undertaking, and depending on the game being played, it's possible to have numbers as big as the largest PvE raids. If you had to choose just one of these two gameplay styles, how would you be spending your MMO time?