Obviously, I would say grades since there is more chance for a good career whether in medicine or something else than for making it in sports. Plus, the life span of an athlete is so much shorter, since everyone seems to think Bret Farve had to choose between a nursing home or playing another year. Actually, I have worked with several retired pro athletes who went to med school after and are very successful physicians so it is not mutually exclusive, just a succession. Lots of physicians have second careers but going to med school and training is like having babies - it is possible to do it when you are older, but you probably do it differently.

Actually, if the return for effort is what you are using to choose a career, many of my physician friends have kids who are interested in acting and the kids make several times more per hour than the parents. Plus, actors have a union that keeps them from being mistreated, guarentees their wages depending on the job and I'm sure many other advantages. Keep that in mind.