Wouldn't this question have to be posed from 2 different areas? One area would be the content and question difficulty, and I assume that the Boards would win out here. But another would be the difficulty of other factors, i.e. what is at stake by the test. Here it would seem that the mcat might be harder, because at stake is the admission to medical school. Most osteopathic schools have a high pass rate for the Boards, if you study wisely then the odds of passing should be highly in your favor. If you don't do well, then there might be some embarassment and you'd take the test again. A poor score might keep you from a prestigious residency, but you would probably get a residency somewhere. You will be a doctor either way. But the mcat has the potential to deny that opportunity outright, so the pressure could be greater.