Mongoose, hands down. I used to be part of the *consistency* school, but here is what you have to consider. The whole idea of duel goose is to get the very best uptime on mongoose - NOT to get duel goose. It is a myth that off-hand procs 24% less only - this applies to both main and offhand white hits. Mongoose is still procing from white attacks and poison proccs on offhand at -24% + your hit. Off-hands procs are around half again, so for arguments sake, that drives is up to 1.5ppm. Or around 33% uptime a minute. 20 agility is never going to do anything for you, statistically it might save your neck 1 in every 50 fights. Mongoose, however, is such a huge jump in defence (10% dodge) and offence (7% crit + 120ap) that it can change games.
Any rogue worth his salt in this bracket is running with tigerstrike + stealth, gem-studded belt, possibly engineering goggles + hit cap. Our damage is gimped to start out with (most rogues nowhere near 500ap and 20% crit), 20agi is not going to bring us up to a competitive level. Most of our damage should be coming from bleeds, and control. When mongoose does proc it puts us well over 300dps, so we can actually hemo hard. It's not usually the AP contribution, but the crits that we notice.
Any priest who has had the pleasure of a rogue hitting on them with duel mongoose knows it's pretty much game over. It won't happen often, but when it does most rogues are looking at 30% crit, 40% dodge, and well over 600ap - that's arms warrior level.