Our favorite w/our 5YO boy is Typhoon Lagoon; He loved playing in the wave pool, and the shark pool (He didn't snorkel, but there's an underwater viewing area), kid's areas, and floating around the river.
BB was fun too, it just seemed like there were a few less things we could do together, and a few more big fast waterslides. If your kids are really good swimmers, they may prefer BB, but there are good slides at TL also. I don't remember if there were height restrictions on the bigger slides because we didn't
We've never been to River Country since we usually travel in the winter when they are closed.