Oyster shell is a better source of calcium. A hen would need to consume the equivalent of 2-3 whole egg shells to provide enough calcium for one days worth. And the size of the particles matter as the hen will digest it overnight as her body creates the shell for the next days egg, and the larger the piece, the longer it lasts. Here's an article that explains more:
http://www.the-chicken-chick.com/2013/08/for-strong-eggshells-size-matters/
http://www.the-chicken-chick.com/2013/08/for-strong-eggshells-size-matters/