I picked A because it seems like it could be used for more than one look, if you know what I mean. It could be for business, for a fancy party, or you could make it casual. Depends on the accessories you dress it up with. Black also goes with anything.
Honestly it depends on the occasion, if you're going to do something really business formal like a presentation or you're going to work and you have a huge meeting then the second dress. BUT if you're going to like a cocktail party or you just want to wear a black dress then the first one. Both are beautiful but it definitely depends on the situation.