It's hard to put a straight damage proc and an attack power proc on the same scale, because the DPS you get from additional AP will vary by the specifics of your gear at the time.
That said, the proc of DMC:D is only 40-50 DPS. Some quick and dirty math says that's not enough to match the Mirror. Twice that might do.
For the trinkets my guild has access to at the moment, DMC:G > Fury of the Five Flights > Pyrite Infuser > Mirror of Truth > Loatheb's Shadow. DMC:D is below Mirror somewhere, so there's no reason to go for it - it's not a high-priority equip, and Mirror is easily available.