The eternal debate of "which water park is better" is pretty subjective, of course. I always like to point out "why" I like TL better. I'm a pasty-white redhead, while my wife is a sun-bunny. BB has far less foliage. Also, we're 40ish, and I've found BB to be more geared towards a teenagers. Not that BB isn't for adults, but it's more attractive to a 15 year old than TL for some reason.
TL has dense trees, and many, many, many places where my wife and I can sit next to one another and have her in the sun and me in the shade. And Castaway Creek, to me, is one of Disney's greatest inventions since the Cinderella's Castle. The wife, on the other hand, will sit in the wave pool for hours.
What we like BETTER about BB: First and foremost, if you're into raft rides, the large raft ride at BB is untouched, as versus TL's raft rides are kind of short. Also important: The ski lift at BB is much less taxing than the hundreds of feet you'll walk in a day, up cement steps, to do several rounds on the TL slides.
In the end, for us there are more plusses with TL. I think your 8 y/o DD will be very happy with either, as will you!
Pat
Pat