Panzar'Thar Breastplate - Item - World of Warcraft
Vengeful Gladiator's Plate Chestpiece - Item - World of Warcraft
I'd have to say it would depend on the rest of your gear. Are you at the defense cap? The SBV on panzar is really nice.
On the flip side you get more armor, probly more stam depending on your gemming and +hit, and the res helps with tanking also.
Honestly its a really close call dude, you could always use veng. for your threat/solo set and panzar for your mitigation set.