They both look great on you - so, I can’t say one over the other.
But, I would recommend to consider a few things when making your decision. 1. Will you run into difficult or more alterations with one dress over the other? 2. Did you have a vision for a shorter or longer train? It looks like the 2nd dress has a much longer train? Either way - you’ll look beautiful!!
Also, will you have trouble with undergarments for either of the dresses? It looks like you definitely won’t be able to wear a bra with the 1st dress (which doesn’t bother some people, as you can have cups sewn in)
Do you have any family members who wouldn’t be happy to see as much boobs as the 1st dress shows? I don’t mean that in a rude way :/ Just something I thought about when I liked a plunging neckline dress.