The statement that is best supported by the details in the passage is:
Both Snowball and Napoleon are persuasive leaders and clever animals.
The passage describes Snowball's ability to win over the majority with his "brilliant speeches" and his knowledgeable proposals for improvements, indicating his persuasive and clever nature. At the same time, it highlights Napoleon's ability to canvass support quietly and effectively, particularly among the sheep, demonstrating his own persuasive skills and cunning. Therefore, both characters exhibit qualities of persuasive leadership and intelligence.