Both.
When PvPing you generally want to be in defensive stance unless you are completely sure of your ability at the assassin class, this will provide you with some defensive and avoidance.
Don't wear leather, you'll get owned hardcore.
Shields? For an assassin? If you want to do one handed dps roll a guardian, that least that way you'll be able to tank as well.
AA wise for assassins... regardless of how enticing they look with all their pretty little knock backs and stealths, stick with the old stand by of 4-4-4-8-1 str and 4-4-4-8 int. Not only will you do the best dps provided for PvP, but if you decide to raid, well than there is no reason you shouldn't be number one!
I threw that extra AA point into wisdom for thost casters, fyi~
But in regards to the title of this thread, kill them before they can attack you and you won't have to worry about either~