I’ve been skiing for 45 years and snowboarding for 25. I teach both. As many others have pointed out, the first day on a snowboard is usually harder, and harder on your body, than skiing. Some people pick it up quickly, but most struggle and fall a lot during the first couple of days snowboarding. Children under age 6-8 (depending on physical maturity, and fine motor skill development) often have a lot of trouble learning to snowboard. Yet they often have no trouble at all picking up skiing – assuming they have an instructor to mimic. Snowboarding takes an interesting combination of good core strength and fine motor skills in the feet/legs. Most little kids just haven’t developed this yet. There are plenty of exceptions, but my kids were great skiers and knew how to skateboard, but they really couldn’t learn to snowboard effectively until they were 7-8. Overweight people should stick to skiing. Unless you are fat and fit (pretty rare combination), you’re going to wear yourself out very quickly hauling your titanic tushy off the snow over and over. Most fat snowboard students don’t make it past lunchtime.
As I said, I’ve been skiing almost my whole life and snowboarding for half of it. When I learned to snowboard, there were no instructors and it took me 4 days to get comfortable getting down the hill without constant falling. These days, good instructors can cut that learning time dramatically. Whatever you choose to try, take some lessons. They aren’t expensive and they’ll make the whole experience much more enjoyable and rewarding. Oh, and on a selfish note, remember to tip your instructor. For context, I’ve taught full-time for one season and part-time for five. Many other instructors will have worked with more students than I have, but my several hundred beginners over the years provide enough of a sample that I think most instructors would generally agree with the below. On the flip side, I’ve seen another 2-3 who just did not ever “get it,” even if I worked with them, hands-on, for three days. Never got up off the ground on their own, never slid on their heel edge for more than a few feet without falling/sitting down. There is, of course, a bell curve between the really speedy learners and the ones who never get it. The majority of new snowboarders will feel fairly comfortable turning on easier runs within 2-3 days. People who just go out and ride without a lesson will immediately start trying to turn on a real run, but I’d wager that it actually takes them an additional day or two to get to the “comfortable” stage. There’s something to be said for learning the basic movements first.