I ski partly because thats what I was first taught, I also don't like having my feet straped down to one board. I think one of the biggest reasons I don't snowboard is because I would not be about to strap and unstrap every time I get on and off the lift. When I get off the lift I want to start right away, any time wasted means I'm not blasting down the hill. I also don't exactly care for skiing in the terrain parks, too much waiting. I would much rather be challenged by the steepness of the hill, trees, mogols, basically the natural stuff, than doing tricks.
I don't really think either is harder on your body. They are both pretty brutal when you are first learning but beyond that its just a matter of how hard your ski or slide.