Why? The only difference is in the strenght of the attack, the claw will always be S 6, with the biting blade varying between S 4 and S 8 depending on the amount of hits, and against a stationary vehicle he will have the maximum amount of hits thus resulting in more likelyhood of destroying it. I'm not sure how you handle the modifiers on the damage chart, but power weapons are not ap1 (they just do not allow armour saves) so I don't think they get any bonus when rolling the damage against vehicles, I'm actually not sure how to resolve melee attacks against vehicles, off hand it seems they should always get -1 on the damage roll since there are no melee attacks I know of that have an AP value and thus should count as 'AP- weapons' attacks, but I'm not sure about that last part.