There's also what's feasible.
This is a photo of the 27 foot deep main display at the Florida Aquarium in Tampa, where I used to volunteer as a diver/algae scrubber.
There's no feasible way to have a real reef. Think of the lighting, skimming, and labour intensity required, as well as fish disease management... if it were real corals. Just not feasible. Public zoo and aquarium standard practices is to use fabricated displays. There are a few large live coral reefs as well, but those are very ambitious and rare exceptions. I saw this one at the Georgia Aquarium in Atlanta (the one with whale sharks), kudos for accomplishing this display, but it's rare to see the really big tanks go with live corals.
more details on what's involved in this live reef display:
http://reefkeeping.com/issues/2008-03/feature/index.php