Well, like everyone before me has said, you need to PvE to gain item and experiance which would be unattainable in PvP. PvE also has dungeon crawls which are truely amazing.
Now, if you mean the difference between the servers, then PvP is for tho…
The game is better after about lvl 15 because thats when ur class really gets definition. Plus that is also when u start doing instances. All in all, i have to say i really like the game, mostly for the atmosphere that the game creates.