It's easy to mangle yourself on both kinds, but it does seem easier to get a severe knee injury on the skis. Skiing is definitely a sport where excellent conditioning is crucial. I can rip either kind of board(s) with good style for about twenty turns, and then things start getting ragged, and less stylish.

It's awesome to see a Tahoe-local ripper destroy a burly section like it was groomed and then keep bombing it out of sight. Either sport is worth mastering, if you have the time and some King-Cash.
The motocrosser has hurt me way worse than the skis ever did, but what finally steered me away from skiing was the crowds- I'm not fond of standing in lines. These days, if I'm jonesing for some snow action, I'll bum a snowmobile from a friend, or take off on the snowshoes for a mountaintop picnic. You can keep those $100 lift tickets- I'd rather buy tires for the dirt bike!