Ok, here is my two cents.
The mace, is a metal or metal headed club fashioned into a flat impact surface to inflict mass blunt force trauma to an area impacted. Some maces have flared points or flanges to increase the trauma area, and also inflict cuts. A morning star is and was a round ball on the end of a 4 to 8 foot shaft covered in spikes to increase the physical damage done to an enemy. This weapons was used on horseback and by infantry to knock off knights and men-at-arms off horses and inflict heavier damage than a mace alone. In many cases, the morning star was wielded two handed, much like a spear or other polearm weapons.
The flail is a weapon derived from simple farming tools, converted into military use like many tools for urban and peasant militia from the 8th century to the 17th. The first war flails were simply two lengths of iron shod wood connected together by stout rope or metal chain, and the weapon continued to evolve as warfare became more advanced. Some medieval guy decided that iron shod wood was really silly, and decided to dump the wood in favor of steel. Another guy thought that the flail the second guy designed wasn't butch enough, so he added another head to the flail. However, he found out (after losing a bunch of teeth due to the second head moving in the other direction) that the shaft of the flail was too short. Unfortunately for him, the third guy to continue the path of flail advancement killed him with a rock. This guy, having more ingenuity, lengthed the shaft to about 8 to 10 feet, and this gave him the leverage to swing both heads at an enemy, and it worked much better than his rock. The fourth guy to advance flail development really liked spiked balls, so he killed the third guy with his morning star (and the third guy's rock was nowhere to be found, so the fourth guy had to develop the third guys flail). He found he didn't like the two stout ends on the shaft, and replaced the 8 ft shaft and its two heads with a 2 to 3 foot handle and a spiked ball connected to the handle. He really liked this flail :twisted: ,and called it a chain flail or mace. However, the fourth guy was known for his skill with a morning star, and he could never convince any of his buddies that the chain flail was a different weapon.
The fifth guy, who was the brother of the third, found his dead brother on the battlefield
, along with his family heirloom, the rock. He grew extremly angry that his brother and his rock were covered in blood, so he developed the new flail. This flail also had a spiked ball, but it had two of them, and as all guys know, two balls are better than one. So the fith guy tracked down the fourth, who had earned a knighthood for killing the third guy, and they engaged in a battle of spiked balls on chains on the ends of a stick. However, they failed to notice the sixth guy involved in the development and abandonment of flails, who shot both of them with a couple of longbow arrows, and the two died without knowing whose flail was bigger. The sixth guy laughed at the two dead foes, until his skull was caved in by the third brother of the third guy who developed the original two headed flail, with his family heirloom, the rock.
All of this goes to show you that no matter how much technology you develop, it still sometimes comes down to which guy has the rock and the other guy who doesn't. :twisted: