Besides from always choosing the bigger group for a better distribution of aggression, I would go for the 20 one inch juveniles. They will grow up together and will not engage in hard fighting from the beginning on. Such fights will happen at some point, but later and in a group that has had time to form some sort of hierarchy.
With the 12 adults the risk for deadly fights is much higher because they are all matured territorial fish and everyone will (have to) fight for their position or sometimes even just for their life until a hierarchy is established.
Which means bigger risk of losing fish and ending up with a too small and therefore unstable group.