Armor penetration or agility? I’m going to give you the rules of thumb.
- Never go ArP if you’re not MM.
- If your gear has less than about 350 ArP from gear (not including gems or a trinket proc), then you should use arcane shot taking improved arcane shot talent, as well as gem agility.
- If your gear has more than 350ish ArP (this amount changes based on how much crit and AP you have), but less than 850 (again, not including gems or a trinket proc), then you should continue to gem agility, but drop arcane shot from your rotation, and use the improved arcane shot points somewhere else. To see whether this is a DPS increase, you need to use a DPS calculator for your spec and gear.
- If your gear has over 850 ArP (again, not including gems or a trinket proc) and you don’t mind being a bit of a “one trick pony” (a term Tobin at EJ coined to mean that you don’t AOE or DPS while moving well), then you can gem ArP and will get a DPS increase.
These are not going to be accurate for everyone, but they will be most of the time. I’m going to reserve the bottom of this post for exceptions to the rules.