4 DKs or 4 Mages?

[waffletastic]

Hi, I currently just leveled my mages to 55, but I've heard Death Knights are very good in PvP. And PvP will be my focus when I get to endgame. I was wondering which class I should choose to go forward with? What I like about mages, is you press 2 buttons, Arcane blast, and arcane barrage, and you can mow through a lot in PvP. But how hard is it to play Death Knights? Which class is more fun, and which do you prefer?

So the question is, 4 Arcane Mages, or 4 Frost Death Knights?

Edit: Also, whats the deal with runes? I've never made a DK before.