It is whatever you play better, but in my experience(I haven't gotten Assassin or Juggernaut, the mirror classes, to level 50 yet)Assassin/Shadow tanking is more complicated. Mostly because of the sheild you must put on yourself every few seconds, I had trouble with that at the beginning but got better at it. Juggernauts/Guardians have the same taunts to make stuff attack them as the Assassin/Shadow. Juggernauts can always use Force Charge, but with Assassins you can't pull things to you until the mid 30s, and you still don't get the whole mob. I honestly prefer the Assassin because I play it better, but Juggernaut is just as good.