Special Forces. Even though both are trained differently, I would take Special Forces because of the mental pain they have to undergo. Athletes would sulk, suck their thumbs, and ask for raises if they had to do the same training as a SEAL. They would never make it because while they may be strong physically, they are weaker mentally.
The real answer is... a soldier of the special forces training as an athlete at a military college such as west point.
It depends on what types of athletes you are talking about - and what type of special forces.
Are we talking about a pro-golfer, or a pro-runner? A Navy-SEAL or an Army Ranger?
In general, I would be inclined to go with the special-forces as being "tougher" in an all-around sense. Their training is more well-rounded for survival and endurance in a variety of environments versus athletes who tend to train under more controlled conditions.
Athletes.
There are different types of shape for different types of requirements.
Athletes are in better shape for their sports and would not do well in hostile environments for several days, and special forces could not do well in sports.
