I have never been to Volcano Bay either but my family always preferred TL to BB. I think it has a lot to do with the theme. BB looks like home to us. If I want to ride a chair lift all I have to do is head on over to the ski resort that is 10 minutes from my house. But palm trees are a novelty for us. Also, we always felt shade was more plentiful at TL than BB and we are pasty white northerners. Shade is our friend.
It is true, Summit Plummet and Slush Gusher are at BB and are fun but I'll tell you, the bigger I get the more those slide hurt my back. I didn't notice it as a kid but as an adult I really do. Humunga Kowabunga is shorter than Summit Plummet but the idea is the same. No matter which you choose be sure your bathing suit is securely fastened and that you keep your ankles crossed and squeezed together tight. Wardrobe malfunctions are not uncommon on these slides.
The lazy river is longer at BB but I think we all still prefer the lazy river at TL. Like Klayfish mentioned the wave pool at TL is really cool. Do NOT underestimate the power of the waves though. Also, water shoes are good, the bottom is rough.
In years past I would have said the family tube ride at BB is much better than the one at TL but Misadventure Falls at TL is a blast and has the added benefit of taking you up to the top in your tube. You get in the tube at the bottom and the life guards steer you over to a conveyor belt that takes you up to the top. You can't put a price on that lol.
Crush n' Gusher at TL is a blast. That is our favorite water park ride anywhere.
Really though, either one is good. If you are water park people you will have a good time at either.