Which is easier to learn for a beginner—skiing or snowboarding?

It depends on how you look at it: It is easier to get reasonably proficient in snowboarding in a few days.  You can be having good fun, cruising blues, and looking decent on a snowboard after a week for sure. But your first day and a half on a snowboard will inevitably involve many painful falls.  Skiing takes longer to master, but you can at least have a fun on your first day out.  It doesn't have that painful break-in process that snowboarding demands.

Snowboarding is easier to learn, but the process is more painful. A good snowboard instructor can get almost anyone down a blue run in a day; if not a morning. I think the main reasons for this are: Side slipping on a heal edge is so much easier than side slipping on skis. This makes it easy to at least slide down a mountain and build up confidence. Confidence is crucially important on first day or two in a new sport. Skiers can go straight which is easy, but then they have to know how to stop which is not. This can lead to skiers going out of control early on in the learning experience which leads to loss of confidence. The stance is far more fixed, so it's easier to get right. No skis to cross; which is a big problem for beginner skiers. Using the upper body to initiate a turn while you are learning is fine in snowboarding (and feels natural to beginners). This does not work very well skiing. While easier, the falls people have snowboarding are more frequent and generally painful. Skiers learning can generally fall to their side; but snowboarders catch an edge and land on their bum or face first. This hurts more! I'd say it's harder to be an expert snowboard than skier if you compare the two directly. Skiers have advantages over almost all terrain which just makes it easier to become an expert; rails and boxes in the park being the one obvious exception.

I have skied for the past 17 years, and learned to snowboard this year. From what I can say, and my instructor reiterated constantly, is that skiing is something that you can go out and do the first day and have fun. It is easy to learn, but very difficult to truly master. Snowboarding is something that is difficult, and painful to learn but much more easily mastered than skiing. I personally will not go back to skiing after learning to snowboard. If you have the right attitude and are able to relax on the board you'll be boarding blacks within a week. I was able to make it down my first black without skating the third day on a board. It's a complete mental game at first, then it becomes completely natural once you log decent mileage.

Many of the people have responded that snowboarding is harder to learn initially but easier to master.  I think that's wrong.  Snowboarding is a quicker, more painful (in the short term) path to the intermediate rut.  But I think a higher percentage of snowboarders stay stuck in that rut.  Once you learn to sideslip a heel edge, you can fake your way down stuff that requires real skill on skis.  Many people that do that think they are good because they can get down a "black" run.  Very few can control speed with turn shape on a carved edge on that black run.  To me that's an important part of expert riding or skiing and it takes a good deal of practice to get there no matter the equipment.

I think skiing is much easier because the beginner's way to turn (the pizza wedge) is foolproof (although it looks uncool). Then, the transition to the (cool) parallel turn is gradual and depends on aptitude, coordination, aggressiveness, balance &c. The point is that the beginner turn can get you down blue slopes with little falling or sitting on the snow. You also have poles so you can walk out of the flats. Yet it may seem inelegant and feel scary until you have mastered the parallel turn. Snowboarding does not have a beginner turn. Beginners need to get a sense of balance and gliding straight or sideways as well as figure out how to make their boards turn. There is no safe method. The alternative is to fall or sit and get back again  (the "leaf" is not a turn, the board's ends remain on the same opposite sides of the fall line; yet I concede that going back and forth on your heels without turning is the boarding beginner's foolproof way down a slope and feels a lot safer than having to point the pizza wedge directly downhill as one is turning). Learning snowboarding involves falling a lot on your knees and tush and being sore (and falling hard, no poles to slow you down). Also, on the flats (and the lifts) you need to unbuckle one foot and push, then buckle it up again, which is a pain (and while you are unbuckled exposes to risk of knee twist injuries but that is very low since you are just walking). Once you are buckled in, though, you look and feel as elegant as advanced boarders. My kids and I have tried both and we are firmly committed to skiing.