I think the question is not whether Protadin or Warrior is better, but how the warrior is compared to the current "main" off-tank, the Death Knight.
In my opinion a good warrior can do the Offtanking job of a DK much better than any DK and i've seen alot skilled DKs in my guild. Simply because of the better Warrior-CLasses Utilitys, e.g vs Shambs they have a much better Dev-CD Synergy (Crowd Control, Last stand + 4setboni Bloodrage vs hardenraged SHamb etc.).
The question is more whether u go Pala+DK or Pala+Warrior instead of this arguing about which Class is better