Sad to say, but 50,000 is not REALLY all that big.. The biggest indoor tank at the Texas State Aquarium (which isn't really ALL that impressive as far as world reknown aquariums go) is 144,000 gallons (trivia I had to know when I was a volunteer there). It's nice, but nowhere near big enough for dolphins, orcas, etc. They have a couple of 5 foot Sand Tiger sharks and a couple of 500lb Jewfish Groupers though. Along with a myriad of reds, snappers, etc. that you would find under an oil rig in the gulf coast (the tank replicates the artificial reefs that oil platforms become)
Now, as to the question at hand, If I had the unlimited money that a tank that size would consume, I'd fill that bad boy up with Live Rock and spend the rest of my days collecting reef fish for it. Everytime I dive I'm alsways amazed at the tiny micro-world that can be found on any square foot of reef... Big fish, sharks, etc. are cool and all, but I'd go for a square tank that would represent a patch reef.
Edit:
So High: Remember that kids read this forum... watch the language please...