Power Weapon or Lightening Claw

With the 5th edition codex, power weapons and single claws have the same cost. Simple analysis suggests that the more base attacks you have the better the lightening claw will be since the lost attack for not having 2 weapons is a small portion of the overall number of attacks. The claw will also work better with higher Toughness targets.


The following numbers are from simple mathhammer of averages:
For T3 targets without a charge bonus the claw is 88% as effective as the power weapon.
This is the only time the power weapon with bonus attack is better.
With Charge on T3 and without charge on T4 they are equal.
Charging a T4 target makes the claw 112% as effective.
It goes up to 137% for charging a T6 creature.

These numbers are for veterans with 2 base attacks, a sergeant with the claw who is not a veteran will far worse. For a captain or master with 3 base on average the claw is always equal to or better than the power weapon for normal targets(ie not grots).

For the full distributions consider 2 cases:

Being Charged by WS4 T3 opponents:
Wounds Caused______0____ 1____ 2____ 3
Power Weapon ___29.6%_44.4%_22.2%__3.7%
Lightening Claw __30.9%_49.4%_19.7%___ 0%

Charging WS4 T4 Opponents:
Wounds Caused_____0____1____2____3____4
Power Weapon __31.6%_42.2__21.1__4.7%__0.4%
Lightening Claw__24___43.9__26.___5.3%___0%

So for offensive minded unit against anyone but Eldar, the Lightening Claw is the better buy. Defensively they are pretty even with the decision based on the toughness of the assault units of the enemy.

So for your vanguard unit in the Rhino with Pedro, a Lightening Claw is going to be better than a power weapon.