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Why do a Design Patent w/ or instead of a Utility?

By way of background, ornamental designs that have only aesthetic, visual qualities are protected by Design Patents, and inventions that have functional usefulness that solve some kind of problem are protected by Utility Patents.  It is common to protect the function or manufacture of a structure with a Utility Patent, and the appearance of the same structure with a Design Patent. Generally, design patents protect the same or obvious variations to the look of something. So, the functionality (i.e., utility) of the invention will not be relevant.  However, the scope of protection should be thought of as the range of similarly shaped (e.g., styled) and ornamentally embellished items.  The protection may be thought of more about what an artist would change to get a certain look, and not what an engineer would do to achieve a certain function.
Generally, if you believe you have a unique function that you want to protect then you should do a provisional/utility application, and if you believe you have a unique look that you want to protect you should do a design patent, if you want to protect both a novel function and look, then you would want to do both budget permitting. 

There is almost never a conflict between Design and Utility patents because design patents do not disclose functionality, only appearance, and are somewhat more narrow in scope.  When applicable, you can grab much broader protection by getting a Design and Utility patents.  That is, in court, there are completely different sets of law that governs infringement of Design patents, so where the Utility patent might fail to win infringement, a Design patent could win by different legal means.

Design and Utility patents are practically mutually exclusive of each other except when the claimed function in the Utility can be completely understood by the drawing in the Design patent.  When your invention's functional novelties are very much tied to the structural shape of the design, a Design patent is a perfect complement to the Utility, and they are much cheaper and easier to get awarded.  Moreover, recently, their patent protection strength has been greatly expanded in patent law.  And they operate under completely different infringement law, so where a Utility might fail on technicalities, the Design patent could win, and vise-versa.