This is a very poisonous thread
Not poisonous mah.. It only bring out the BBB virus in us..
Both are glass material, drop both tell me which one won't break? The idea is - handle with care no matter what.
Well there's advantage to both round and square. Round you can fix it up comfortably at home or in a safe location before going out to shoot. But square means technically you can use it on ANY lens within the maximum width constraint.