Why would you worry about crits in heroic MgT?
Nvm it's not important. The point is, yes, defense is always better. Yes, it is easy to reach 540 to an extent. Yes, if you don't quite make 540 it's perfectly ok to wear pvp gear to make up the small amount of crit chance left over. I still say there's way too much discussion on this.
I tanked all of BT up through our first 2 Illidan kills in a season 3 chestpiece, it had tons of stam, armor and hit rating, not to mention strength. I used the 18 stam/20 resil head enchant too, imagine that. And guess what? It was fine.
"There are no men like me...only me."