Nightcrowler had a good write up about armor pen. This was based on the PTR data before the 2 days of nerfs (rip and tear crit bonus to FB down, rake dot no longer crits, SR glyph down to 3%)
I highly suggest you read the entire post.
Link:
FeralbyNight (Cat & Bear simulation tool)
Info:
Stats value
Stat points dps value
Armor Pen (shred) 28.4 +/- 0.1 1.55 +/- 0.01
Agility 26.9 +/- 0.1 1.47 +/- 0.01
Armor Pen (RIP) 26.8 +/- 0.1 1.46 +/- 0.01
Feral AP 12.00 +/- 0.1 0.65 +/- 0.01
Strengh 23.8 +/- 0.1 1.30 +/- 0.01
Crit 22.9 +/- 0.1 1.25 +/- 0.01
Hit 22.0 +/- 0.1 1.20 +/- 0.01
Expertise 22.0 +/- 0.1 1.20 +/- 0.01
AP 10.00 +/- 0.1 0.55 +/- 0.01
Haste 18.5 +/- 0.1 1.01 +/- 0.01
Discussion
The best in slot trinkets are grim toll and the agility darkmoon card.
Shred idol is better than RIP idol, with the same setup you will do 7179.22 DPS (+3 DPS) but not by a large margin. Shred idol scales better with ArPen while other stats scales about the same way (that's why I've putted 2 row for armor pen).
It's important to notice that all stats scales about lineary in relative value. All but Armor Pen, the more you have, the better it is.
There are 2 boundary lines about ArPen:
At about 400 ArPen with RIP idol and 200 ArPen with Shred idol, ArPen becames a better stats than agility.
For a similar reason at about 240 ArPen Shred idol becames better than RIP idol.
Edit: All info above taken directly from post. I wasn't sure how to put it in a quote box to make sure credit was given where due.