Hospital tanks and QT tanks are completelly different in there own. One is for observation and one is for treating. Alot depends on what you are wanting to treat as spanky said. Acros, fish, LPS and what you are treating for is all a major factor IMO. A hospital tank for fish should not be used for a hospital tank for corals.
IMO opinion a hospital tank for fish can be the same as the QT tank. However on SPS I would rather have two setup. One for observation and the other for treatments. Most treatments on coral are short term doses. It would be much easier IMO to observe the corals and then if need be move them to the hospital tank to do the required treatment. LRB's is something that could be treated in the QT tank but AEFW would be better off treated in a seperate tank. My fish get treated in the qt tank as I find it much easier to treat fish regardless. My fish will get treated for ich, worms etc, regardless of weather they show signs or not. Just is a easier way to give me piece of mind that something unexpected is not going to pop up in the display tank and infect the other fish.