Backstab Dmg or Ratio?

[Lowlife]

Level 30 Rogue. I currently use a Polished Steel Dirk (8/23)with a Burning Rapier (7/20) in my offhand, should I drop overall ratio of the PSD for a Gloomwater Harpoon (9/31) for more backstab damage? Or, a third option, move my BR to my mainhand for even less backstab damage and put an Electrum Bladed Waki (7/21) in my offhand?

To simplifiy..

Option 1. PSD / BR
Option 2. Gloompoon / BR
Option 3. BR / EBW
Option 4. You tell me :P