Kinda comparing apples to oranges.
Soccer is very physiologically demanding, but not to the extreme that pro cycling takes it. You're never gonna dig as deep in a game as you would at a mountaintop finish or you wouldn't be able to walk let alone pass the ball accurately by the end.
But soccer takes a much more rounded and disparate skillset to compete at a pro level, it just does. Complex muscle memory (ball control and striking), spatial awareness (foot/eye coordination as well as a broader awareness of the position of other players on the field) and tactical thinking (race radios have widened the gap here for cyclists) are all much more important in soccer. Couple this with the general rise in athleticism (read: doping) of pro soccer players in the last 20 years, it seems to me that soccer is "harder."
Also, in terms of numbers of participants vying for the top slots in each sport, soccer has cycling beat by miles (or kilometers). It's much more difficult to become a pro soccer player because you're competing against millions kids who have literally been playing for hours a day, 7 days a week for most of their lives.
Though, to some extent no matter what the contest, regardless of inherent difficulty, world class competitors will always make winning really really hard for each other.
As an aside, it would be pretty interesting to see some pro soccer players take up cyclocross on a lark. Probably the closest thing to a soccer game in terms of physicality, plus they would destroy those run-ups.
Kinda stupid, cuz he's talking about baseball vs. wanna be tri geek, but this is kinda true.http://www.youtube.com/watch?v=De7rbB2bteE