If you've ever skateboarded or surfed, go with the snowboard.
Skiing is easier to learn, but you will still be going very slowly for those three days if you've never done it before. I enjoy skiing because I've done it much longer, but my best experience was on a snowboard in a foot of fresh powder...omg.
You will fall less in skis but its really hard to maneuver around flat areas unless you take them off, while if you are snowboarding you take one foot out of the binding and "skateboard" around.
Oh, if you have ANY problem with your wrists DO NOT snowboard because when you fall forwards (1/2 the time) you will land on your outstretched arms. The other 1/2 of the time it will be on your ass/head. My brother-in-law nearly knocked himself out when we were learning to snowboard.
BTW, I found snowboarding pretty easy to learn after I took the short instruction class they offer - don't try it without that lesson or you will hate it. It is really quite easy after you learn how to turn correctly during the lesson, and it's kind of counter intuitive if you ask me.
tl;dr - My vote, snowboard after a short instruction class if you don't have any wrist problems.