it's really kind of hard to just simply "stack" defense versus avoidance because of the way blizzard itemizes gear. most epic tanking pieces have a mix of defense and avoidance stats on them. as far as gemming is concerned, I've always been of the opinion that once you have the defense cap you should be gemming for stamina to increase your Effective Health, because avoidance kind of comes on its own as gear improves. also consider that avoidance is subject to diminishing returns, even when it comes from Defense Skill. so the more of it you stack, the less benefit you get from it per point:
if you find yourself having to make a choice between two pieces of gear based on raw avoidance and defense skill, just do the comparison on a piece-by-piece basis. if one increases your total avoidance more than the other without sacrificing other stats to an unreasonable degreee, i'd say wear it. I wouldn't say you need to worry so much about stacking one over the other, unless you're worried that you might fall under cap. However, if you're ever unsure, I would say err on the side of defense.