Here's some number crunching for those mathematic genious's ^^

Which weapon would be the best offhand dagger for pve combat daggers?

Here's the basic breakdown of the two weapons:

Maexxna's Fang
Binds when picked up
Unique
One-Hand Dagger
94 - 141 Damage Speed 1.80
(65.3 damage per second)
+8 Stamina
Requires Level 60
Equip: +36 Attack Power.
Equip: Improves your chance to hit by 1%.

Harbinger of Doom
Binds when picked up
Unique
One-Hand Dagger
83 - 126 Damage Speed 1.60
(65.3 damage per second)
+8 Stamina
+8 Agility
Requires Level 60
Equip: Improves your chance to get a critical strike by 1%.
Equip: Improves your chance to hit by 1%.

To break it down even further:

Maexxna's Fang:
80 life
34 attack power
+1% to hit

Harbringer of Doom:
80 life
8 attack power
+1% to hit
+1.28% to crit
+0.53% to dodge

Harbinger is slightly better as an off-hand than Maexxana's Fang.

Harbringer is a crit and 8 agil.

Yeah I'm a nub.

Sarnath'd

This is correct. Harbinger = more poison procs, and its stats yield more dps than MF.