I wrote a
blog article on this. The conclusion I reached was that at all levels, expertise is far more effective than avoidance at reducing your chances of parry gib - twice as good at all reasonable levels of dodge and expertise, essentially. EH is good as well, but expertise quickly reduces that chance. That it also has a side benefit of threat is a great deal for stacking. This is true for any tank, but especially true for warriors and druids.