They have the same basic meaning. However, when we use the singular form, it usually means that this is the order/philosophy/command/rules/promise, etc., of the speaker. So we might say, "In my house, my father's word is law!" We say, "I give you my word" to mean "I promise"; to say, "His word is gold" means you can trust him/believe him. To talk about a person's "words" (using the plural form) means simply to repeat what they said... the words aren't especially important or dramatic.

!!....the only difference i can see is that one is singular and the other is plural. "God's word" or "God's words" (God's) when this form is used, it usually indicates something belonging to someone and in this case "words" belonging to God "words" and "word's - i am not sure one can use "word's" in a sentence, at least i have not seen it being used before