Depends on your stlyle. Mongoose procs quite often, but it might be at a useless time, like on the killing blow in the arena. But when it does proc: ~5.5% extra crit, dodge, slightly higher attack speed and armor (it increases agi by 120 for 15 seconds). If you prefer the reliability of savagery's AP boost that might be for you. I personally love my mongoose enchant for pvp, when it procs you notice the difference.
Once you start to get Armor penetration from pvp gear and rings, you may want to switch to executioner to really stack damage on soft targets. Cloth and leather wearers will drop very fast indeed. I'm not enough of a theorycrafter to know at what level of latent Arm pen you'd want to switch your weapon enchant though.