I have, by coincidence, both a DK and a Hunter at 85. I also, by coincidence, have quite a bit of experience at what you're calling "farming," i.e. hitting pre-Cataclysm instances for fun and profit.
In my opinion, Hunter is easy mode for instance farming. My hunter, using a tank pet, can solo any dungeon up to lvl 80 with few problems. I've even soloed most of Forge of Souls, which was a top-end dungeon added towards the end of WotLK. (The last boss is a problem, and would be for anyone solo.)
As a Hunter with a tank pet, you can kick back and let your pet (who is relatively expendable) take the brunt of the mobs. All tank pets can learn a move called Thunderstomp, which is an AoE, so they can tank packs of mobs fairly well. And your ice trap, Disengage, Feign Death, Misdirection, and Deterrence all make the task of keeping aggro off of you and on your pet easy.
Also, between talents and glyphs, its fairly easy to keep your pet healed even while he's tanking a phalanx of mobs. If you use a Turtle or Beetle pet, they can reduce the damage they take.
So yeah, I'd recommend Hunter. It's got great end-game potential as a DPS and minor crowd-control, and can solo pre-Cata instances like a champ.