The first one is fairly easy to dismiss; it's from an index, and I can see how the mistake was made. An entry for Joseph Noah Abernathy in Ancestry's WWI Civilian Draft Registrations states he was born 20 Apr 1875, but an image of the draft card shows 20 March 1875. So right off the bat two is down to one.
But then there's his death certificate. It gives Noah's birth date as 3-21-1876, and one of his daughters is the informant. Furthermore, his tombstone in Bollinger County, Missouri's Plainview Cemetery offers the same birth year. And we are quickly back up to two.
I do have some extra evidence to consider. The marriage license for Noah and his wife Marry Elizabeth Kirn states Noah was under the age of twenty-one at the time the license was purchased, in January 1896. Unfortunately for my research, both birth dates would bring about that result.
I lean toward the birth date on Noah's World War I draft registration card simply because he should have been the one to provide that information. I guess other arguments could be made, though. What do you think?
By the way, Joseph Noah Abernathy (1875/6 - 1957) was the son of Sion and Margaret Abernathy. He was also the father-in-law to my great grand aunt, Gladys Marie Campbell via her marriage to Noah's son Harry Joseph Abernathy.