Rationalism is unlocked later in the game, during the Renaissance Era. The policies are mainly focused around lowering unhappiness and raising happiness, causing you to go into golden ages faster, which boosts the speed of everything, from building, to research, to gold production. Peity can be unlocked early in the game, since it becomes available in the classic era. Piety can be even more valuable than rationalism, or vice versa, depending on your strategy. There are a variety of different ways to win and for each victory condition there’s plenty of strategies on how to obtain them.
It really all depends on how you want to play the game and how you plan on winning. The truth is that no particular social policy is “better” than another.
