In reply to Lurking Dave:
"Either is easier once you have a good feel for the other."
Disagree! I've been skiing since I could walk, and every few years I would put the effort into learning to board (until a few years ago when I gave up in abject frustration), and whilst I had a good understanding of the physics of the whole process - it was, after all, just a shaped plank being weighted in different ways to aim it where you wanted it to go - I found it very difficult to translate that into which bits of my body I should be leaning, flapping, wiggling, that kind of thing.
Having not had the opportunity to learn snowboarding *without* already being a fairly good skier, I can't honestly say that my proficiency on skis actually held me back, but I do know that it didn't help me as much as I thought it would. Or maybe, just maybe, I'm a naturally shit snowboarder.