After doing both sports, skiing is easier to learn and harder to master (as stated above) while snowboarding is harder to learn and easier to master. (In my opinion).
Most anyone can put on a pair of skis and make it down the mountain while most people that put on a snowboard for the first time sit on their butt for most of the day. Skiing with the correct technique takes a lot more time and practice than snowboarding with the right technique.
Snowboarding is by far the cheaper sport, a good pair of boots and bindings can easily be bought under $200 and a nice board can be bought for $200-$250.
Skiing is a lot more expensive, boots and bindings are $300 plus, and nice boots are up by $450, while nice skis are $600+.
Skiing injuries seem to happen mostly with knees (ligament tears) and hips (unlike snowboarding where its slightly more difficult to land on your side) and snowboarding injuries seem to be comprised of wrist, arm, and shoulder areas.
Hope this helps.