Armor pen is a funny mechanic in that it works in layers.
Basically its something similar to tank armor mitigation. Just to use round figures (not actual wow calculations) it works as follows:
If the boss has say 30000 armor and mitigates 10000 damage. However if you remove 33% of that armor it still might mitigate 9000 damage. If however you remove 66% of the armor it only mitigates 5000 damage.
Basically in order to gem for armor pen I would recommend that between gems and gear you can get enough armor pen to reach at least 70% armor penetration and continue to gem for it once you reach that mark until you hit the 100% hard cap.
Until you can make that 70%+ armor pen figure you should continue to gem for Strength as it will give you a slightly beter DPS return.
As you don't say what relem you are on in your user profile I can't look at your gear, however for a 2900+ gear score your DPS seems extreamly low. You may want to reasses your gear choices, too much or too little hit, expertise, rotation etc. While warriors are gear dependent as a class, they require balanced stats and over doing any of them (with the exception of hitting 100% armor pen) can easily nerf you. Personally I like to run with a minimun of 180 hit (in addition to 3 points in precision), 26 expertise, and a least 40% crit.
