Well, you probably want the leather kind that has agility. You can level in any spec, but if you plan to PvP a ton, go Subtlety. If you're doing the PvEs, go Combat or Assassination. Combat has the bonus of basically allowing you to use swords, maces, axes, or fists, while Assassination/Subtlety require daggers (at least in the main hand, although Assassination requires a dagger in each hand later). If you're 80, go assassination and faceroll to victory with Subtlety.
Basically, EJ is a good idea in theory, but you really have to get to 80 before you can say what YOU want to do. Mutilate is faceroll and you might get bored of it. Combat is interesting. Subtlety is apparently decent for PvP. Just play around with specs every once in a while (do a stint of PvP for several days, then go for a PvE spec for the next bit) to see what you enjoy.