If you have such a trinket and are near the soft cap for ArP i. Otherwise no, keep gemming for strength. The best way to tell is to download Landsoul's spreadsheet from Elitist Jerks, import your character from the Armory, and look at the SEP values it calculates. So from what you say I should keep maxing out on ArP. Still find it confusing.
Executioner enchant as well. Executioner is worse than Mongoose. The Armor Penetration is a nice boost, but remember that is coming at the expense of other stats, be it Crit, Expertise, Haste, or Hit.