Love the dress on both of them, but I agree with dbarn that Victoria won this particular head-to-head.
I think how Victoria styled it played to the delicacy and romantic slant of the dress. The accessories, hairpiece, etc., all worked to make a kind of --- to me --- total dreamy, delicate look.
While there's nothing wrong with the bag, shoes and jacket Mary wore (I thought they were all great pieces, BTW), they added a harder, businesslike edge to a dress that was leaning toward a softer, more pretty, rather than businesslike, look. So, the complete look she had on kind of reminded me of two opposing opinions clashing, in a way.
Maybe I'm not saying it correctly to get my point across. But Victoria's came across to me like one mind created it, versus Mary's coming across to me like TWO different minds having created her total look.