None unless one of them is the width of the fireplace and the white one is plain ugly.

Looks very good as the white looks great through your home but it was a wart looking on the brick to me, light walnut looked the better?! Here's a photo of a walnut with rock, It goes with both brick to rock.?

Ken

Walnut Fireplace Mantel · More Info

I love how you've selected some simple square edge mantles. In your existing photos you've got a low hearth that wraps around the fireplace. I'd emulate the same design by wrapping the mantle around and returning it to the wall. It doesn't need to be as wide as the hearth but it will look better if both shapes are the same. Good luck and if you need to contact me directly for more design feel free to do so via my website contact page!