Personally, I believe that excelling in academics is of more important. Only through academics added on with extra-curricular activity can one best improve one's chances of entering a top-tier college/university. Of course, maintaining one's shape and staying healthy is of great importance too, but excelling in academics is priority for younger people.
Another answer from our community: Religion, specifically Christianity is not weakened by academics but by unbelief. The Christian faith can encourage knowledge and inquiry, but many choose to pursue this in a way which violates Christian belief. Many academics are Christian and pursue their field in a way which, according to the bible, is pleasing to God, gives Him glory and helps people.