What's the difference between cheap pencils and crayons for coloring?
Quality will be much higher with pencils. The softness of crayons dramatically reduces your precision when drawing, and they have a tendency to "skip" - I guess because of the higher amounts of wax involved. This is why crayon drawings rarely have a uniform color field, and don't blend well.

You will use up your crayons much faster than pencils as well, because the soft wax smears much faster. As you mentioned, the wood of the pencils also lends durability. I've never had much luck mixing the two unless I want to see the different marks - the materials are different enough to stand out next to one another.