Simply put, a Utility Patent describes the way something is used, while a Design Patent describes how something looks. The proverbial "better mousetrap" needs a Utility Patent. The ornamentally ornate carrying case needs a Design Patent, unless a radically new development in carrying-case technology is also present--unlikely!
A Utility Patent requires a description sufficient that someone skilled in the art could create it, given the description. A Design Patent almost always requires descriptive images only; in rare cases, some descriptive words may be helpful.
It is not unheard of for a Utility Patent application to run to more than a hundred pages, and more than a hundred claims. By contrast, a Design Patent is at most a handful of pages, and is required by regulation to be restricted to one claim.