Generally you'd look at:
- the age of the questions
- whether or not the questions have answers (you can't close a question as a duplicate of another question that has no answers)
- the quality of the questions
- the quality of the answers
One question is considerably older, they both have at least one good answer, but I'd say the newer question is slightly better written. I don't think there's an obvious closure candidate here, because they're ultimately about the same level of quality.
They also have two (sort of) contradictory answers. DVK presents two possible theories on how Horcruxes function, one which agrees with Beofett's answer and another that would completely contradict that answer. I'd be slightly wary of implying that one answer is more correct by voting a particular way.
All of that to end up saying that I really couldn't decide between the two. Flip a coin? Or leave them both open, nothing bad is happening with them right now.