I would say that a MTB is made to handle rough terrain. It's not really as much about comfort as about the handling and robustness. A roadbike is made to go fast. Both bikes can have a fair amount of comfort engineered into them but generally speaking both of them are about performance. They just happen to each be addressing two different types of performance. A roadbike can be more comfortable than a MTB and vice versa. This all depends on the terrain, how they're set up and the specific geometry of the bike. More often then not, it also depends on the person. I find that the more upright position of a MTB is less comfortable than my roadbike when using it on the road for rides longer than several hours. I can however ride all day long on my roadbike on roads. For road work, a roadbike is more comfortable to me. Now if I was tackling singletrack then I would have to say that a MTB is more comfortable than a roadbike. One needs to qualify their statement when they're talking about comfort.