Hi, Debbie!
Great question!
I always consider where we'll be spending the rest of the day when making reservations of any kind. For example, when my daughter and our friends' daughter got their magical makeovers at Bibbidi Bobbidi Boutique, we chose the Downtown Disney location because we wanted to spend the evening there dining and shopping, too. If you know you'd like to spend the day in Magic Kingdom or Epcot, I suggest the location in MK, as you'll save yourself the extra commute time by being right there, post-makeover.
There are parades at Magic Kingdom, while there are none at Downtown Disney, so that may sway you toward one location more than the other as well. If you have a smartphone, download the free app
My Disney Experience which will allow you to see the times of everything from character meet-n-greets, parades, park hours and fireworks. You'll love it!
Finally, while only Guests between ages 3 and 12 are able to receive a makeover at
Bibbidi Bobbidi Boutique, Guests of any age can receive a makeover at
The Pirates League, which is also in Magic Kingdom (Pirate Empress or Mermaid, anyone?) Of course, mom can also head to one of Walt Disney world's many spas in order to get a makeover of a more traditional kind, too. Nothing says vacation like a fresh manicure and pedicure, right? ;-)
Send more questions as you think of them, Debbie; I love to help!