We have been to both. Coral Reef is really nice and it's cool to sit near the giant tank watching the marinelife (although a bit awkward, as you're sitting their EATING marinelife)...anyways...
However, Biergarten is just a plain good time!!! Besides the fact that you get GIANT beers (which my husband enjoys), there is music, and dancing, and the food is pretty darn good. I would say my kids had a lot better time there than Coral Reef.
Both are great, but I like Biergarten slightly more!