Design Patent may show, yet not claim, functional features

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 extremely narrow in scope.  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.