Wow. All this arguing about which is better. And to think, I just got out of the "Which is your least favorite trait?" where people are discussing how useless the Spiritual trait, and it is one of the frontrunners in the poll.
With Spiritual I can have BOTH Slavery AND Caste System, and reap the rewards of both of them with minimal setbacks. In fact, I love it when I frequently swap civics. Since I can only swap every 5 turns (marathon), after I swap, it's 4 turns of peace and quiet from the AI civs demanding that I adopt their favourite civic.