The use of "will" in the future simple represents a definite plane. If I tell you we "will" do something, we are 100% sure that something will happen.
"Will" is also used for predictions when we are very sure. Example: the weather, Leonardo DiCaprio winning an Oscar, etc.
"Let's" is a suggestion. When something says "let's" +verb, they want you to either consent or decline. One could argue that it used more frequently when the speaker expects consent. "Let's", similar to "will" assumes to a high degree that the proposed action will happen, but unlike "will" requires some sort of yes or no from the audience.