Hi Kristi,
While the actual make-over at either of the
Bibbidi Bobbidi Boutique locations is the same, it is the experience of being in the castle that makes the Magic Kingdom location fun to some people. Granted, it's the first floor, but it's still fun to tell your little girl that she gets to go there. That being said, having her make-over at Downtown Disney and then heading over to the Magic Kingdom all decked out on her first day would be wonderful too.
I think you could spin either one to be the best in your mind. For example, if you kept the Magic Kingdom location, you can tell your daughter she gets to do something extra special on her last night there. She'll leave Disney having just been turned into a Princess. If you book at Downtown Disney instead, you can start her vacation off with an enchanting experience.
Either way would be Magical, it really depends on whether you want to begin or end your vacation with Bibbidi Bobbidi Boutique.
Have a wonderful vacation,
Kaylene