The "which waterpark" is a very commonly asked question. Not at all trying to sound "snarky", just recommending doing a search and you'll find a ton of info. But I'll give a cliff notes version. We love the water parks. Have been to each of them at least 20 times.
As for which one is busier, they really are about the same. I think by sheer numbers, TL is slightly higher, but really very, very close.
Which one do I prefer? Well, that's really a matter of what you want and there's no wrong answer. If you FORCE me to pick one, I think I'd say 51% TL and 49% BB, but I pretty much love them equally. They both have similar attractions...tube slides, lazy river, "big" slides, kiddie areas, family tube rides. Just different themes. Some of the key differences....the wave pool is the biggest difference. The one at TL has waves like you've never seen in a wave pool. They're humungous. If you want to stand in the deeper end of that pool, you need to be a strong swimmer. But if you are, it's beyond awesome. The beauty of it is that even non-swimmers can go in it, just stand in the shallow end...in fact one of the kiddie areas is off to the side of the shallow end of it. The wave pool at BB is very small waves that are constant. Sort of like bobbing up and down in the ocean. BB has a zip line and iceberg crossing challenge for younger kids, TL does not. TL has a "roller coaster" style tube slide, in that jets of water push you up small hills, not just down. BB does not have that. BB has two "big" slides, TL only has one...however, the "big" slides at BB are only one lane each, the one big slide at TL is 3 lanes.
How long to stay? Depends. If you want to chill and relax, you can stay all day. But we usually arrive at park open (10am usually) and are done by about 2pm. The first hour the park is open, you can get on anything you want with basically no wait. The only limit there is how fast you can climb the stairs...there are lots of stairs. By 11am, the lines for the e-ticket stuff start to build. That's the best time to get in the wave pool or lazy river. Lines will stay long until about 4pm, then slowly come back down.