An NDA is a contract of confidentiality, where a patent is a protection of intellectual property. Contracts are enforced differently in various states and very much matter solely on how well it is written if the contract is broken: it will lead to more of a compensation and damage control and hopefully a payout if proven. Whereas patent law is a federal enforcement and is universally enforced in the states and is backed by the government if infringement actually occurs. Therefore it always best to protect your ideas with a patent, an NDA will have some sense of security when discussing ideas with people one has confidence but its best to patent the idea.