Within the thought experiment, the difference is simply numbers and people are giving the wrong answer, as long as you specify that this would increase the total number of years lived (many organ recipients are old and will die soon anyway). Outside the experiment in the realm of public policy, it is wrong to kill the "donor" in this one case because of the precedent it would set: people would be afraid to go to the hospital for fear of being killed for their organs. And if this was implemented by law, there would be civil unrest that would more than undo the good done.