Great Weapons on an Ironbreaker specced for greatweapon mastery tactic = 20% more damage than a 1 hander.

Great Weapons are fine for the burst dps you get from them versus a 1 hander in PVP. Over time your 1 hander will catch up, but if you're sitting and pounding on a guy for 15 seconds then you need to find some friends to assist you. If the shield is REALLY saving you a lot of damage as a tank in PVP then either you aren't getting trained or you have no healers healing. I die to assist trains, not a situation where a shield is going to save me a lot.

IF you are running around solo continually with no healer then sure, use the shield. IF you are never gonna spec into greatweapon mastery or are an SM or aren't on an assist train then sure, use the shield if you want. IF you're doing PVE and need more survivability then sure, use the shield.

Just don't confuse the "damage over time" equals out with a 1 hander versus a 2 hander trick. If things die faster, then you aren't fighting as long, then the 1 hander has less time to catch up and "even out" the damage. If you have multiple people assisting a target then you want max dps on that target so it goes down faster. Let your healers worry about keeping you alive and you worry about dealing damage. As tanks we have more armor, more health, and more survivability than melee dps, but we also have the ability to get closer to melee dps damage if we want to. Don't pigeonhole yourself into thinking one way is the only way to do things.