I use a DK currently. If I had my time again in the current game, I would have levelled my pally from 70 to 80 instead. Pallies at the moment are the best tanks threat and survivability wise as well as being the easiest to multibox.

Saying that, when I was deciding on a tank, unholy DK had the awesome bonus of Ebon Plague for the spell damage bonus as well as having awesome survivability. What hasn't changed however is that pallies are by far the easiest tanks to create effective macros for one button tanking.

So my recommendation at this stage would be for you to use the pally considering it is also your highest lvl tank, but keep in mind, the best option at this stage, may change down the track when the game has its next "tweak" :P