I've done both and I'm pretty good at both, black runs etc.
Skiing is easier, plus with the poles you can push yourself along slow runs. You have better speed control on skis and you don't have to sit down once you've stopped, so no getting cold.
Snowboarding is more difficult for a beginner. If you're on slow runs (Blue, Greens) and going slowly which no doubt you will be, every time you stop, you will have to unbind your snowboard and push yourself one footed, which takes a fair amount of balance to do, which I don't think you will have first time. As you can only stop on your front or back edge on a snowboard, it's more difficult to control your speed especially in deeper snow and on ice in comparison to skis. Also with a snowboard, if you stop on a hill, you will have to sit down on the cold snow to stay there ^^ With skis, you can just use your poles or angle yourself correctly.
Also drag lifts on a snowboard are pretty difficult, even for me!
Falling is also a issue. With ski's your binded in but can snap out of them when you fall, and so reduce injury

With a snowboard, your strapped in completly, so if you fall forwards your chest will hit the deck hard and your legs will kick that snowboard up behind you and you'll boot yourself in the ass haha
If your on the top of a slope and having difficulty with skiing, you can slowly snow plough down and control your speed well. With a snowboard, if you're having difficulty that slope will be a ****ing nightmare and you'll get demoralized about being in the snow much quicker.
I would highly recommend skiing over snowboarding, even though I enjoy both. For a first timer, skiing will suit you much better.
Hope this helps, sorry for the wall of text! But it has a lot of information in there
