Hey Chris, I am sorry to hear that you argued with your mom. That is never fun. I am doing a TTD session in my real wedding dress. You only get to wear it once for a few hours, why not have fun and get all the use out of it you can? I was going to buy a used one on Ebay to use but I really like my dress and I want pics in it. If it will save an arguement buy a cheap one on ebay or look for a cheapie one in your area. I found one in our local Pennysaver for $50 but I had already pretty much made my mind up to use the real one. The majority of the brides on this site who used the real dress said that it was able to be dry cleaned and turned out just fine.
Bottom line, I think that this is your time to be a princess. This is the one time in your life where the only people who need to be pleased are you and your FI. I think we have to be sensitive to others, but really you need to have the freedom to make this your event. Anyway, after the wedding when your mom sees the pics she will more than likely be happy that you did a TTD. If it will save an argument buy a cheapie to use. That is one way to keep the peace.