Well, dual potency is a static 80 ap, mongoose has negligible haste and 120 ap with an approximately 50% uptime, maybe better. Dual mongoose = 25% both / 75% one / 25% neither uptime, 8%/4%/4%/0% crit respectively, an average of +6% critical strike.
Current work in the itemization thread ranks 1% crit as worth approximately 38 ap, that meeans 6% crit = 228 ap. So dual mongoose is 185% better for DPS than potency just comparing crit to AP. Mongoose also provides minor haste and significant dodge.