Just to end the "They use the same stats!!!" argument, I took it upon myself to do 5 seconds of research. As it turns out, mastery is the best stat if your tanking followed by crit. For gladiator you want crit and haste, and mastery (the thing you want to stack as a tank) is way at the bottom of the list. Bonus armor is only found on jewelry and cloaks, and to be honest are the only pieces of gear that can be optimal for both tanking and dps at the same time.