From having skied both downhill (competitively) and cross country I believe downhill is more challenging. Cross country is about strength and endurance. Downhill is about strength, endurance and agility. There's very little room to screw up in downhill. Aside from wiping out, there's a reason skiers wear heavy padding on their arms in slalom.
Go figure my son isn't listening to me when I told him to take it easy on his snowboard this year (junior year) due to baseball. He threw it back in my face I skied competitively on weekends in high school and screwed off doing freestyle with my friends (no flips). Then he threw in the jumping off bridges and cliffs in the summer. Is it possible to convince a kid because we did something doesn't mean it was sensible? Had there been half pipe boarding when I was a kid that's where you would have found me.