Response A is better because it follows the prompt instructions more closely and provides more clear, correct, and complete information for planning a themed costume party. Response B is also helpful, but it includes more unsupported assumptions and focuses less directly on the costume theme ideas.

Response A does a better job following what was asked and gives clearer, more accurate information for organizing a costume party with specific themes. While Response B offers useful tips too, it makes more guesses about things not mentioned in the original question and doesn't focus as much on the actual costume theme suggestions.