It is one or the other, but not both - and Nexus validity will depend on the itinerary (eg: closed loop to/from the US)... So as others have said, one is usually better off with a passport. I have Nexus and there has been a few times when boarding a place where US CBP insisted that all guests show an actual passport when doing secondary inspections of all passengers at boarding. CBSA recommends to always travel with your passport, regardless of your Nexus membership in case you need to prove citizenship (or permanent residence documents).