As a rule, I think PvP is the more rewarding experience, as people tend to have the ability to stratagize and improvise as they go along in a single battle. In any PvE experience I've had, no matter how 'well' done the AI, it still fell into predictable patterns, and while challenging, never provide the same level of ingenuity and determination found in a PvP setting.

But then you end up with games like Diablo 2, where the PvP just wasn't worth the 30 seconds it took, so there's always an exception to the rule.

I mostly agree with Pxzero. I also think PvP brings the best grouping experience in most MMO's or at least it can be ^^.

There is only one thing you need to remember. I believe that the 'helping' and 'achieving' aspect of PvE is important in any MMO. A PvE approach mostly gives you these aspects and most PvP experiences don't.