Medicare Advantage plans — private plans that serve as alternatives to the traditional, public program for those that qualify for it — underperform traditional Medicare in one respect: They cost 6 percent more. But they outperform traditional Medicare in another way: They offer higher quality. That's according to research summarized recently by the Harvard health economists Joseph Newhouse and Thomas McGuire, and it raises a difficult question: Is the extra quality worth the extra cost? It used to be easier to assess the value of Medicare Advantage. In the early 2000s, Medicare Advantage plans also cost taxpayers more than traditional Medicare. It also seemed that they provided poorer quality, making the case against Medicare Advantage easy.