Why Ski Board's skiing is easier to learn
From a beginner's perspective, skiing is easier to learn than snowboarding. Why? Because it's more intuitive. This means that you go forward and downhill in a manner similar to walking, running, or hiking. Obviously, skiing is not as easy as walking or running, but since your body is oriented in the natural direction, it is easier for beginners to learn than snowboarding.
Snowboarding is still very natural and I know many people who have learned it quickly and easily. But if you try skiing for the first time one day and then the next day for the first time, I think most people will find it easier to ski, overall. For beginners, the basics of skiing are also more accessible. You can become a good beginner snowboarder in a much faster time than you can be a good beginner snowboarder.
You can learn how to make a basic pizza wedge turn in minutes, and you can easily get off the bunny slope without worry on your first day. That is, if you have a good mentor anyway. While it takes a lot of skill and experience to become a professional skier, learning the basics can be accomplished very quickly. Skiing is obviously also easier if you have experience skating or cross-country skiing.
Why Snowboarding is Easier to Learn
Even though skiing is easier to learn if you have never tried skiing before, snowboarding is easier to learn if you have experience with other types of snowboards. The basics of turning a skateboard or snowboard are very similar to snowboarding. So if you've ever played either sport, chances are you'll learn to snowboard more easily than skiing.
Snowboarding is also easier to learn because there is less gear involved. All you really need are your boards and boots. You'll also need warm weather and protective gear, but that's technically less than what you'll need to start skiing. It also makes it cheaper to get all the equipment, which can be a significant barrier to entry that keeps some people from trying the sport in the first place.
Skiing involves using both legs on two planks, while snowboarding involves using both legs on one plank. This can make snowboarding easier to learn because you don't have to think or worry about your legs moving in opposite directions. They stick very well to the board on your feet. This means your skis won't fall through the wreck like snowboards.
Snowboarding is also easier to learn in terrain parks. If this is the type of skiing or riding you want to be good at, you should consider learning about snowboards first. It also comes down to the one plank vs two plank debate. In a terrain park on a snowboard, it is often easier to hit a feature, such as a rail, than on a snowboard.
Should I try both?
If you're just starting out, I'd really try a day or two on snowboards and snowboards. You may find that you prefer one of them, or that you are naturally good at one of them. If you've made up your mind about which one to pursue, I'm not trying to convince you to drop it. But spending a few days on both is a good experiment. I learned to ski and snowboard at a very young age and I'm grateful I did.