For super-selective school admissions, the answer to “A in easy class versus B in hard class?” is likely context dependent.
For example, if choosing the easy class tips the overall schedule out of “most demanding” or leaves an apparent subject deficiency (e.g. stopping foreign language at a lower level, not taking all of the basic sciences, etc.), then it is likely to be more harmful than choosing a hard class and getting a B. On the other hand, having more than a very occasional B grade is likely to be disadvantageous as well.
Students who make the “A in easy class versus B in hard class?” (instead of “A in hard class”) choice frequently are likely to be uncompetitive in admissions to super-selective schools.
Of course, the choice of classes for the purpose of preparing for actual college work is a different angle to this question, but there is also some context dependency in this angle as well.