Depends on the system mechanics; 3.5e/1e that first one will have way higher AC due to Natural Armor, Mage Armor, and the inevitable Monk dip.

As to the real world, so long as breastplate doesn't have a concave catch point at the underboob the second would be both practical and effective. The riveted decorative "cleavage" at the top is actually fairly common and practical for diverting arrow/lance splinters away from the head and neck.