I have another question, if the dress I want which is beaded and lovely turns out to be fancier then the one that the MOB picks do I have to change my selection to a less wow gown? This is my one chance to wear a lovely gown and I really don't want to take a huge back seat to the MOB. I am not trying to show her up but I am just wondering,
It sounds like you already know the answer. The MOB chooses her gown and the MOG coordinates hers in formality, style, length, and color. You would not copy her dress, just coordinate. You would definitely not dress more formally than her.
I agree with my colleague, sorry about that. Why don't you talk to the mother of the bride and see what direction she plans on going with her dress. Maybe you can both wear something fancy.