Ich würde Einschreiben sagen, außer wenn im Text beide Versionen vorkommen, dann ggf. mit Fußnote. Offensichtlich wird Registered Mail gesondert behandelt und besser beaufsichtigt.
Hier die genaue Erklärung des Unterschieds vom Web Site der US Post:
Q. What is Certified Mail service?
A. Certified Mail provides the sender with evidence of mailing and an electronic record of the delivery date and time or attempted delivery. A signature is required at the time of delivery. A copy of the delivery record that includes the recipient’s signature is available at your local post office for an additional fee. Certified Mail does not provide tracking service. It is available for First-Class Mail and Priority Mail articles only.

Q. What is Registered Mail service?
A. Registered Mail is the most secure way to send First-Class Mail and Priority Mail domestically and internationally. Registered articles are insured and placed under tight security from the point of mailing to the point of delivery. It provides both a receipt showing an item was mailed and an electronic record of the delivery date and time (for items delivered in the U.S.). A signature is required at the time of delivery. A copy of the delivery record that includes the recipient’s signature is available for an additional fee.

Q. What information will I receive on the Internet for Certified and Registered Mail?
A. You will receive delivery or attempted delivery information.

Mit registered mail kann man auch einen am Brief befestigten sogeannnten "return receipt"bekommen, eine Postkarte, die mit der Unterschrift des Empfängers an den Sender zurückgeschickt wird. Mit Certified mail geht das nicht.