Stam increases the value of armor, armor increases the value of stam. Now it is true that armor has a DR curve, but it caps out at 45000(it is 45000, right? or is it 40000... I forget explicitly, pretty sure it's 45000 though) which is 75% reduction. Until you get around 72% or so the DR isn't going to be THAT steep.
It really just all depends on your current levels of stam and armor which one is better. If you have low armor but a really high amount of stam, armor may end up being better, but if you have really high armor but a low amount of stam, then stam is going to be better.