The U.S. requires the priority date to be within one year of the first public disclosure or offer for sale. These are substantially arranged in order of cost. Sound practice is to choose the particular mode of protection with a cost benefit analysis and choose counsel based upon reputation of the firm or lawyer. An NDA can usually be obtained at little to no cost. This certainly is the cheapest method of protection, but is fraught with problems.
People refuse to sign the agreement.
Seeking a signature delays or stalls out the marketing process. Copies of the agreement are lost or the inventor forgets to have it signed. One mistake has serious consequences, making the tool an unsatisfactory choice in most all circumstances. In essence, the PPA is a cost shifting tool that allows an inventor to determine the market worthiness of an invention before incurring larger expenses. The priority date can be carried forward in a subsequent FPA or PCT application.