Colleges want to see two things on a student's transcript: high grades and challenging courses. Which matters most? For highly selective colleges, both. It's nearly impossible for a student to get into one of the top colleges or top universities without straight "A"s and tough courses.
Students who really want a shot at a highly selective college can't think in terms of high grades or challenging courses. They need to get high grades in the most challenging courses available. An "A" in gym or wood working isn't going to impress a top college, nor is a "C" in AP Calculus.
The best advice, then, is to take the most challenging courses available and put in the extra effort to earn a high grade. Realize, also, that there are many other benefits to taking AP courses.
As a final note, it's important to realize that students don't need to get straight "A"s in tough courses to get into 99% of the colleges in the country. Places like Harvard and Williams are not your typical colleges, and in general a few "B"s or even a "C" won't destroy your chances of getting into a good college. Also, students who struggle with AP courses would probably find themselves in over their heads at the country's most selective colleges.