And like him, I'd like to reiterate something - something that Runy stated recently too: please use brain.
Everyone's gear, playstyle and needs are going to be different. Yes, armor pen is projected by a number of theorycrafters and simulations to be the best stat to stack when you get 'enough', where 'enough' is a wide-ranging number. And yes, I've given a rule of thumb for when you should make the switch. But whether that's best or not for you really depends on...you. There are so many variables that you can't reasonably answer it all, but here are some common ones:
- 10 vs 25? If you're not getting a ton of raid buffs regularly or do smaller raids, chances are armor pen isn't going to be as solid due to not having as much base AP.
- Bear or cat? Armor pen is a good threat stat for bears, but it doesn't hold a candle to agility for overall usefulness. If you're going bear a lot and have a lot of shared gear, don't use arpen in it!
- Hit/expertise? Armor pen tends to do better as you're closer to the hit/exp cap, mostly because it benefits from less spiky behavior - and misses contribute to local spikes.
- Crit rate? If you've got a low crit rate, the chances of getting bad cycles are going to be much higher (where you're missing uptime on rips, etc). If you're not at at least 50% crit rate I'd recommend not bothering with arpen no matter what you can get it to.
- Types of fights? Fights where you can do more burst damage tend to favor agility; if you need to pile in a lot of damage quickly, agility often wins out by sheer luck. Fights with more sustained DPS are wins for arpen. ToC has both at various times; it very much depends on your role.
Honestly, I think Jacemora's going to be disappointed because he simply likes how agility feels better compared to armor pen. It has nothing to do with whether or not arpen theorycrafts better than agility and everything to do with that personal feeling.
That personal feeling is not something you can theorycraft. It's up to you to figure it out.
In any case, both agility and armor pen are very, very good. What I would really recommend is that if you are on the fence, pick one or the other. Don't waffle and hybridize. High crit rates make some really interesting cycles possible; high arpen make some amazing damage possible. A mix of the two makes nothing special.