How do you decide whether you should ski or snowboard?

Try them both, and try them both for multipule days. Rome wasnt built in a day just as you dont become proficient in a day. Snowboarding is easier to learn at first if you have no skate or wakeboard experience, but it opens some possibilities that snowboarding cannot touch. I found snowboarding relitivley easy to learn but its much harder to become an expert. Skiing is also easier to learn if you have roller blading or ice skating experience, really helps with keeping your balance centered. Both can provide sensations that cant be felt in any other sport. Also dont get caught up in the skier boarder battle, this is a fabrication of tourists and newbies. There is no battle anymore.

Personally I would snowboard.
It just looks more fun to do.. And it LOOKS easier than ski-ing, I don't know weather it is or not though

Whichever one your good at or trying to learn.

You just choose one, it doesn't make a difference. You're going to have fun either way so if you want to try one over the other, go for it. If you can't decide, flip a coin.

Personally, I would go for snowboarding. More of my friends snowboard than ski, snowboarding is viewed as more "hip" and "young" than skiing, and I'm more comfortable with the footing of snowboarding than I am skiing. (I haven't actually been skiing or snowboarding yet, but I have been sand boarding and skateboarding. Fun, fun, fun.)

try them both then decide which 1 you like more ya nob-end! simple