We love them both! But they offer very different experiences:
Biergarten is a boisterous, social experience where you may be seated with another family, and where the live music is upbeat and calls for some audience interaction (yelling out a toast, etc.). Of course, if the music doesn't get you out of your seat, your need to visit the buffet will! If you are comfortable chatting with others and enjoy serving yourself, diving into a big stein of beer and joining in a song now and then (and/or if you have small children and want to go somewhere that they are sure to find foods they like and won't bother anyone if they're noisy), Biergarten is for you.
Coral Reef is quieter and more intimate, with the only "entertainment" being the fish swimming lazily by in the aquarium, and where your food will be brought to you at the table. (If you opt for Coral Reef, ask for a seat in one of the bottom two rows, and tell them you'll wait longer if necessary to get one. The closer you are, the more immersive and fun the experience.) If you'd rather only chat with your own party, hate buffet lines and prefer a quieter, more upscale-style experience, Coral Reef might be more your speed.
Also, a note about the food: although the difference in serving style makes it difficult to compare the two, having eaten at both multiple times, I have to say that the food quality is equal between the two restaurants -- there are hits and misses both places, although the food at both is very good on average. What gives the edge to Biergarten, in my opinion, is that if you get something off of the buffet and find it lacking, you have twenty more options from which to choose. If you don't care for your entree at Coral Reef (which happened to me on my first visit there), you're stuck.
