Note: As of patch 4.0.1 armor pen is no longer available as a stat on gear or gems.
There has been an increasing amount of talk of the value of hunter armor pen vs agility. The whole armor penetration vs agility debate started, as these things tend to, with the spreadsheet theorycrafters who found that at a very high level of armor pen, and with the right armor pen trinkets, armor pen would actually be a better stat point for point than agility.
What’s the Armor Pen Cap?
Right now it’s about 1400 armor penetration. That’s right, 1,400. You now need a crazy amount of armor pen for it to be worth your while, on the other hand our top end loot now has a crazy amount of armor pen on it, making it possible to actually hardcap.
Is Armor Pen Better Than Agility?
For the majority of hunters the answer is no. Agility is better than armor pen (for MM and SV). You should not use armor pen gems, and you should not use armor pen food – you should stack agility. You will do less damage stacking armor pen than if you use agility/attack power in those slots. Obviously BM hunters prefer attack power over agility or armor pen.
However, at a certain high gear levels armor pen does actually surpass agility and you can do more dps by stacking armor pen. The exact number varies depending on your specific gear (’cause there’s more to it than just the ArP) but figure in general somewhere around 800 passive ArP rating from gear (not procs) is needed before armor pen becomes better than agility.
But wait, there’s more:
Because armor pen only benefits our physical shots and not our magical shots, the armor pen versus agility rule only works if you are MM – who have a lot more physical damage. Furthermore, since most of our physical damage is in the form of autoshot and steady shot, we only get the full advantage on fight in which we can stand still a lot. The armor pen advantage works best for hunter who are very skilled at managing their movement (and managing movement is a key skill for all hunters). And you will want a special talent build to take advantage of that (with Imp Steady Shot and no imp arcane) and remove arcane shot from your rotation except in special circumstances like running shots when nothing else is available.
You can see more details in the WHU MM Armor Pen Build guide.
And even then, if you’re in a fight where you have to move a lot or need to manage multiple target dps (or aoe) the armor pen route may yield lower dps.
How Much Better Is It?
If you can get the gear, the armor pen vs agility is significantly better if you can manage your movement well. You can be competitive without explicitly optimizing for it, however.
Amor Pen Spreadsheet Theorycrafting Error
When the armor pen vs agility was first discovered, it looked like a big deal; however, it turns out the spreadsheets were in error. They were set up to average the armor pen proc from trinkets. Unfortunately, this doesn’t work because once you have that much armor pen, you can actually hit the armor pen cap with the trinket proc. Due to averaging the procs the spreadsheets ignored that, giving it more dps than it could really get.
The error was discovered and the spreadsheets were corrected, which significantly nerfed the armor pen vs agil theoretical damage. Then armor pen was nerfed 20% in patch 3.2.2. It still shows to be better than agility (in the right conditions) but now you need to get up to the hard cap to get the big big benefit.
I recommend stacking agility instead of armor pen except the highest levels of gear. An agility-build hunter is more versatile and can maintain high dps on the move — something that armor pen builds suffer at. And as you well know, most endgame fights involve a lot of moving :)