There is a clear difference between a patent and a NDA. First of all a patent is a legal document that gives you or a company a legal binding of ownership that is reinforced by penalties. The NDA is a contract that is limited. If we are comparing the two by law, a patent is far more power than NDA. NDA seems like a good security blanket but not a good idea for a new product or idea. NDA is only strong as the contract. It is possible to find loopholes in the contract. For example, two people sign a contract for a new device. One person tells his friend about this idea and they start to develop this device. There is nothing you can do because the contract is between you two not the friend. You can sue the partner but not the friend. So patent is the better route.