Om Mani Padme Hom. It is Buddhist tradition to believe the Truth and not who said it.Truth(not the person) is the most important.Supposing Nagarguna had established a religion with a teaching going further than the Buddha's preaching in the Hinayana(though now only the Theravadins of the Southern Buddhist traditions remain as an independent school), then we should believe Nagarjuna and not the Buddha, since the former taught the ultimate and complete truth