Athletes are good at sports and athletic things, but outside of that their skills aren't necessarily useful for human life. I think nerds are just better equipped, in this day and age. Being physically fit is hardly a requirement and doesn't give as many advantages as being mentally fit. Basically, athletes will be better than nerds at sports, but nerds will probably be better than athletes at most other things.

Ultimately though I think you would have to figure out what metric is most useful for determining success. Humans essentially exist to reproduce, and athletes are probably more likely to reproduce more frequently. Then again, nerds are probably better equipped to raise children to be successful in this day and age.