Well as for a new tank i would go pally for a few reasons
1. heals/loh.
2 more gear options then a DK (aka healing gear or ret gear for os).
3. more armor with the use of shields and block takes less dmg
4. Pally buffs
5. Crusader Aura faster group speed for getting around

Now dont take me wrong I have a dk and will use him at sometime in my group as a tank but i enjoy the pally alot enough to by a Traveling Tundra Mammoth for it.