Hello, Sandy!
We've learned, over the years, that we literally can't do all and see all in one Walt Disney World Vacation, so learning to choose is a great skill to acquire.
Both shows are wonderful, and each one is filled with a unique type of Disney magic. In order to choose one over the other, first decide if Belle and Beast are more your little one's cup of tea, or if seeing a multitude of Disney characters in one place would excite her more. A description of the Magic Kingdom show, and a list of those characters appearing,
can be found on Walt Disney World.com's Dream-Along With Mickey page.
Next, consider the venues. The Magic Kingdom show is at Cinderella Castle, which makes for fabulous photos, as well as a dramatic set for this particular production. If you're there early enough, you can sit on the pavement right in front of the Castle, and hope that folks don't stand up in front of you. If you get there a little late, you might need to find a spot to stand to watch the show. (I have LOTS of pics of our little one sitting on Dad's or Grandpa's shoulders to see this one!) In Hollywood Studios, however, Beauty and the Beast-Live on Stage is produced in a covered theater, with plenty of bench seating available. If somebody is really tired and in need of a break, this is a great place to relax as you are entertained by the story so many of us have come to love.
Beauty and the Beast-Live on Stage is described on WDW.com, as well.
I hope that this helps you make your choice between two very enjoyable, but very different, shows! Either way, you've bound to enjoy it!