Better or merely different? While a flail WOULD take a lot of skill to handle effectively compared to a mace, we may assume that a competent warrior using a flail would have these skills if he wished to survive for long. I have no doubt that to use a flail, especially mounted, required great skill, as with many specialty weapons. But in the right hands, as with any other weapon, it would be something to avoid getting hit by, especially at speed. A solid mace transmits the shock of impact down the shaft to the user, while a flail insulates the user's arm somewhat because of the linkage. This particular example of mine would have a much quicker recovery time because of the short chain, and it is also unlikely to bind, get tangled up or wrapped around something, again because of it's short length and the unique chain link design. A mounted man with a flail with a short chain is less likely to injure either himself or his mount if he is trained properly in it's use and isn't just swinging wildly about. And if he is using it properly, the impact due to the the momentum of the swinging ball rotating backhanded coupled with a charging horse would far exceed the impact of a mace. But a mace is an easier to control weapon, having no moving parts. These are different weapons, the mace and the flail, and are used differently, and as such it is hard for me personally to say which is "better."