I would tend to believe the birth date from a birth record, over a birth date from a death record. Census records are often estimates and not accurate either.
The birth information on a death record is most likely provided by the informant, who may know have known the correct date and guessed.
Also, the dates engraved on headstones are often incorrect as well. Again, provided by people who didn't have access to official records, or typos and mistakes by the engraver, and no money to make the corrections.
On the other hand, if dates are far enough off, be careful that you're not actually dealing with different families of the same name and locations.