I personally like mastery. I think it goes great with expertise/hit max. But alot of people will tell you to stack haste instead of mastery. I think both are viable.
Depends on what you have for item and what the dk have for item imo. If the dk is sitting on 522 ilvl and you are sitting on 496/502/509 then it is a bigger upgrade for you imo. Atleast the belt. The neck I would think twice on as necks and rings do not have any armor. So it is not as important upgrade for you as other items. So the belt is more "free game" than the talisman imo.