Majority of my work consists of preparing a utility patent. Every now and then, I get to work on a design patent, but then again, it's not that often.
If you don't know already, utility patents give patent owners the right to prevent others from making, using, or selling the invention. Utility patents are granted to inventions that provide new and useful results. Design patents, on the other hand, protect an ornamental feature of a functional invention, and the ornamental feature only. This means design patents don't protect the functionality of the item.
When seeking a patent protection, should you file a design patent AND a utility patent? Most likely, the answer is no.
I rarely advise my clients to file both a design patent and a utility patent. Sometimes, I advise my clients to file for a design patent instead of a utility patent. I do this when the invention is very simple, and the client tends to focus on an ornamental feature of the invention. For example, I was preparing a utility patent application for a mouse pad that looked like an animal. Well, there wasn't anything special about the mouse pad. So I asked the client if she wanted to file for a design patent instead of a utility patent. I ended up saving her money and my time. Situations like this is pretty rare, and I can't advise many people to do the same thing. You really have to consider what the invention is about.
Sometimes, the ornamental feature of the invention can be borderline functional... Say, for example, a really cool looking door knob. In these type of situations, I might tell the client to think about filing both a design and a utility patent application. Why? Well, if the utility patent application does not work out, then at least the design patent application can be a nice fall back. Mind you, design patent applications generally take less time to register, and they don't require maintenance fees. So clients usually agree to this. But again, this doesn't happen too often.
Whether you should file for a design and a utility patent application really depends on what your invention is. And even then you still have to consider the purpose and intent of your invention, and what kind of rights you are seeking.