Both have incredible appeal and different challenges in place. Your options are more limited with a reef in terms of the fish you can keep because of the number of fish that would turn your reef into an all-you can eat buffet, but the aggressive fish tank is also a challenge because of the wide variety of behaviors you have to accomodate in terms of feeding and how careful you have to be in planning for size. Honestly, the tank that holds the most appeal for me right now (other than a peaceful mangrove setup with refugium attached to accomodate exotic syngnathids, deep-sea pygmy angels, shrimpfish, possum wrasses, cowfish, and mandarins) would be a deep-sea reef with peaceful inhabitants, with anthias, fairy wrasses, deep sea butterflies (like Tinker's and Mitratus), pinecone fish, flashlight fish, squirrelfish, basslets, and perhaps some cardinalfish and gobies as the piscivorous inhabitants. That would be a spectacular setup in my mind...versatile, colorful, peaceful, yet full of action.