Well, it depends on what your research objectives are. ISS is in some ways a better model, in some ways a worse one. It's better in that it's in space with microgravity, but ISS crew members rotate in and out. Even if individuals spend the equivalent time of a Mars mission on the ISS there will be new faces, a constantly changing research workload, and the ever-changing panorama of the Earth below.
So it's not a very accurate model of the social dynamics of a Mars mission where people are cooped up in a can with the same faces, same scenery, and nothing but busy-work to keep them occupied. Let's say we lick the radiation and microgravity problems; the question then becomes what kind of people can successfully negotiate the trip to Mars, arriving ready to work successfully there?