I would say that soccer takes more coordination, and is more difficult in many aspects. But in terms of toughness, I would hands down say XC. It's so grueling, and there are so many times when you want to give up and walk, but you can't. The mental aspect to xc is such a huge part, because your team isn't relying on you in the same way as in soccer. Your team still relies on you, but the pressure to do well comes more from within than from outside pressure.
So soccer is more technically difficult, like coordination and such. But xc is more tough. Also, in order to be a good xc runner, you need to know how to pace yourself which is fairly difficult.
I used to run xc, and played soccer a little when I was younger, so I know much more about xc than soccer.