This is a good question. It is very difficult to say which city in 420 was stronger. I will assume that you are also talking about military strength, since they were at war. In the year 420, Athens and Sparta are evenly matched from a military point of view. There is no clear winner in the Peloponnesian war. Athens is the naval power of the time and Sparta is the infantry power of the time.
In Athens we see the rise of Alcibiades in 420. He is elected stategos, which means general. And the year before this, there is peace with Sparta (the peace of Nicias). This should tell you something, namely no one is really clearly winning the Pelopennesian War.
As for the internal structure, Athens and Sparta are very different. Athens can be seen as a radical democracy where every male gets his fair share of leadership. Sparta, on the other hand, is more of a military society that is ruled by two kings, 28 elders that gave council and limited the power of the kings, and a number of ephors to keep the kings in check. In the end, Sparta does win the war, but the two cities were evenly matched. But from a historical point of view, the west embraced an Athenian democracy or a variation of it.