one theory is that neither are dead, that moriarty faked his death, and that Sherlock knew he would, and so the person falling from the hospital building wasn't either of them, but a dead body made to look like sherlock, courtesy of Molly his pathology girl, and all the surrounding people, medics, were friends of hers. I did watch the first time and so not sure either, but it was good all the same. On the other thread there are some very plausible clues and theories.
Apparently steven moffatt has said that it was easy to spot how it was done.