I would dual-wield Mongoose.
It's just worth more in terms of AEP than +15 Agi.
At the moment, I dual wield +15, but that's because my daggers still aren't quite good enough for Mongoose, and because I'm an enchanter and can do +15 Agi, but not Mongoose.
I chuckle when i see Rogues dual-wielding Crusader, because +15 Agi is better at 70, then Mongoose is better still. (Although, to be fair, some rogues wield Crusader because they can get the mats cheaper than +15).