Hi Sun
I am having the same dilemma! We have a wedding to go to when I'll be almost 9 months pregnant which is also black tie.
I'm not sure either of those dresses will be ok for a black tie dinner, unless maybe you could wear a smart jacket over one of them and dress it up with jewellery?
I'm getting a maxi dress from Tiffany Rose for the wedding, then going to dress it up a bit.