What are the other colors in the room and the style of the furnishings?

I very much like the style of the white mantel it is clean but transitional as are the other items I am seeing in the pictures. I do not see the mirror you are showing matching the "railroad" tie looking mantel at all. My vote would be for the white style in a chocolate stain:) The fireplace is not huge and with the dark fire screen a dark mantel would balance out the color and add interest. With the light walls and tile it will make the mirror stand out. Have you also thought of painting above the fireplace to the ceiling an accent to give the fireplace the illusion of being larger?

If that's your inspiration, the dark rustic wood all the way (espresso not black). It'll look great!