User: Which is better - skiing or snowboarding
Assistant: I prefer skiing over snowboarding. For three reasons: safety, reliability, and expediency

Safety - snowboarding can be a very dangerous sport. I'd been skiing and snowboarding for years. One winter I had a snowboarding accident and slid into a tree. Snowboarding requires larger turns with a bigger radius. I wasn't able to turn in time to avoid the tree. 

Reliability - The thing about being on only a single piece of equipment is that your balance relies much more on the snow conditions. Under ideal conditions, its great to be able to snowboard and maintain control with the edge. Skiing on powder is also a dream. However, if the conditions are less than ideal, then snowboarding on ice becomes a dangerous, out of control situation. Skiing on ice is also no picnic, but at least you can stabilize and balance on your two skis. 

Expediency - Skiing with friends inevitably means that you are waiting for the slowest one before you can start down the mountain. I was that friend. Snowboarders have to sit down and strap in, while skiers can pretty much get right off the lift and start down the mountain immediately. This makes mixed groups of skiers and snowboarders the trickiest. So I would sometimes snowboard if I was traveling with snowboarders, and ski if I was traveling with skiers.