I understand where you are coming from, I have had some of the same problems with my parents wanting to do things their way. I felt really bad because they are helping pay for this types of things, but I have had to tell them that payment does not mean it is there day.
I would say in your case you should express that it is ultimatly your day, and I don't know your mother, but I bet she will understand that the one you want is more your style and better yet an expression of yourself.
I know parents help pay for a lot of weddings, and I know that this is common, but if you haven't thought of it, think about a gift for your mother to show your gratitude for helping out with the wedding.
I know this might be hard, and this is just my opinion, but I would say that you should pick the dress of your dreams, and not regret something on your wedding day. Hope this helps.
Its your wedding, its your dress and body. Do what you want
