Either French or Ballroom will look great. The design on the back of the train is not specific to either style.  You get to choose.  Don't let the owner dictate what you have to have on your butt.  It is only her opinion.  If you had less design on the upper section of train I would say a Ballroom bustle would look better.  If you have more design on the upper portion I would say a French bustle would show it off better.  But your design is equally spaced down the whole back.  I think a French bustle would be lovely if that is what you like best.