The written word is an expression referring to language can be read, whether on paper, computers, stone, clay or any similar medium. It does not refer to any specific words but rather to the manner of conveying the language - by writing words down.
The spoken words, as opposed to the spoken word, is a reference to specific words. It means that words that were spoken by someone, whatever those words might have been.
The spoken word, on the other hand, like the written word refers to the manner of communicating rather than any specific words.
Thus you could say the spoken word is more persuasive than the written word, referring to the manner of delivering a message.
To say the spoken words were more persuasive than the written words refers to particular words.
It's the difference between the general statement the dog makes an excellent pet (referring to the species of animal) and a statement that refers to specific animals the dogs make excellent pets.