That's a heck of a question, Rox. I was a skier for a long time. I think it's definitely different, and there are different aspects of each that are more difficult. I think a lot of it depends on conditions. If it's a bit icy, then I feel skiing is more challenging. If it's good powder, then I'd say snowboarding.
However, it could be that I do a LOT more skiing than snowboarding. I know this isn't the usual experience, but I get a greater thrill from skiing than snowboarding. Perhaps because I did it for so much longer a period of time.