Armor = Damage mitigation.
Defense = Increased chances of parry/dodge/block, lowers chance of taking a critical hit or a crushing blow (note: crushing blows only apply when attacked by monsters, and defense above your level's cap has no further reduction of this chance).
The best to use? Enchants :-P
I'd go with Defense as a tank, though... if you really need it. The armor mitigation increase is very small when you have 7k+ armor.