Having flown a lot in both a C172 and a DR40, I find that in terms of passenger comfort the C172 wins by a significant margin. In the DR40 you feel like you are perched on the aircraft, rather than sitting in it; the back seats are really just for kids. The front seat adjustment options, as mentioned by Backpacker, are poor.
The DR40 is, however, rather nicer to fly. I just would not want to spend more than a couple of hours in one.