There is no protocol when it comes to a mother-of-the-bride dress. You must wear what you love and what looks good on you. Generally the bride will dictate what her mother will wear...but today, anything goes.
A mother-of-the-bride "dress" could even be an elegant pantsuit.
Whichever style you choose...make sure you feel great in it. It's going to be a long day and/or night. You want to look great and feel great.