Considering it is 25HM, you are at a slight disadvantage to other tanks (warrior or pally, in particular) because although you will probly hit the 50% avoidance rate with gear alone, the amount of damage going through will be extremely heavy because the only mitigation you have is just pure armor (before raid abilities).
Problem here would be the fact that you're tanking 100% hasted nerubians that slice through your armor with expose weakness (part 1 of your problem) and threat output of Death Knights is debatable when it comes to holding it off your raid DPS with the requirements for it (part 2).
More health, yes but look into other revenues as well. If you can push your dodge to be higher than your parry, it may benefit you more while still keeping that rough 50% avoidance because it scales lower with Diminished Returns than parry.
Threat wise, you can always suggest your raid to take out the other two while gaining ample threat on your targets so you don't feel the need to be overwhelmed with lack of threat on the first bat but survivability can factor in so it's hard to say really.
Hope this helps.
Healing is the source of life. Without it, you're left with death.