The 700 Armor Pen break-point you sometimes see referenced in regard to where Arp overtakes Agi is a gross simplification.
The reality is that Arp is influenced by several things other than the amount of "passive Arp" itself. It works, essentially, as a damage multiplier on all your physical, non-bleed damage. Having very low overall damage, as is likely to be the case at your gear, diminishes the value of Arp.
With 700 Arp, all your physical damage does 20% more damage than it would have with 0 Arp (assuming the presence of Sunder Armor/Faerie Fire).
If you're only doing, say 4k DPS, of which only 2.4k DPS is physical, 700 Arp increases your DPS by something like 0.2*2400 = 480 DPS. If your base DPS were higher, and you were doing 8k DPS, with 4.8k of that being physical, 700 Arp would then provide a 960 DPS increase instead. Whereas Arp provides a purely multiplicative effect (i.e. it multiplies all your phys. dmg by a mathematically determined amount), the DPS gain from Agility includes an additive component - the Attack Power. This is a huge part of what makes Agi better with lower overall gear.
The other reason it's an oversimplification is because it assumes the highly unrealistic conditions of no AoE or movement. Realistically, things like Arcane Shot, Explosive Trap, and Volley are going to be used with some frequency in most of your encounters. These things tip the scale in favour of Agility rather considerably, because they derive no benefit from Arp.