Superior Bird Skull only has 10% defense compared to the original Bird Skull 25%, I'm not sure it's worth it even with the additional effects.

I discovered a trick, you can unequip and equip both Bird Skull's to recover sanity if it is too low, you can abuse this a little especially with the Superior Bird Skull.

In the default settings, both boomerangs are the same thing, I believe that the Superior Boomerang could have a little more range and/or damage in addition to being stackable so that you could throw several at the same time more easily.