If you want your Farseer to assault with the spiders, by all means give him the W.Blade+CCW combo. This is cheaper, you'll note, than the Singing Spear and it gives the farseer an additional attack, which is nice. Against vehicles, the Singing Spear will hit on a 2+ when you throw it, but you get one chance and the spear can only ever glance a vehicle. With the W.Blade/CCW you get three shots on a charge and you have the ability to penetrate the vehicle.
Me, personally, I burn the extra 18 points and give my farseer both the W.Blade and the Singing Spear. That way, I have the good shot at a glance, and then in case that doesn't work for me, I can go for the W.Blade attacks. I don't recommend this, though, since it burns up a great deal of points and leaves your farseer with 1 close combat attack (after all, you always throw the spear in the shooting phase)...