The whole point of guillotining was that the weight of the falling blade would be sufficient to decapitate the accused every time in the most 'humane' and efficient way without room for needless error and suffering, as was in keeping with the new Enlightenment ideals that sort of precipitated it.

Hanging on the other hand, is well known to have been a slightly less than ideal way of execution if you wished to avoid errors. A too short drop could end up in someone being slowly strangled to death, too long and potentially the head could be ripped off altogether. There are the odd story of the rope snapping too and the condemned having to be hoisted up to be dropped again.

It is interesting that you mentioned these two in conjunction though - the guillotine was invented in almost direct response to the room for error that existed in hanging.