If using the digitally sign feature of Acrobat/Reader, there is a special digital certificate that is attached to the PDF that includes a lot of information as when the PDF form was signed and information to tell if the form has been changed and the ability to "roll back" the form to a state prior to the signature being applied.
Fill and sign is a different product for applying a signature to the PDF.
Use of the digital certificate can be used to validate legal or business forms internationally and comply the legal signature standards that have been internationally agree upon. "Fill and sign" has no such standing or ability, so any formed signed by the "Fill and Sign" option should not be recognized as legally binding in an U.S. court or the courts in many other countries. You might find a court that accepts the "Fill and Sign" but that ruling can easily be overturned because of its lack of conformity to international standards and the standard in the U.S. established by the UCC.