Hello avniel, and welcome to the friendly Camera Labs forum!
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The answer is pretty clear:
Focus on the person if that is the subject of your image. And the backdrop look be even better when it is a bit unsharp.
Or focus to infinity if the person is not the major subject but the sunset and you use the person just to lead into the image or give some perspective.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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