I dunno about doubles. The first one I ever saw was done on a snowboard--JP Walker--and they've been doin em ever since. You might be right about park booters, but they're still a hot ticket in the backcountry.
As to the original question: I do both, and they're different. Snowboarding is way harder to learn, but it seems more difficult to execute all the mechanics of skiing properly. That said, you can go days on skis without binning it but ride a snowboard and you're guaranteed to fall at least once a day. So I'd say balancing on a snowboard is harder, because of the single balance point, than on skis. Both are fun, both are good for different things, going fast on skis is just amazing.