Colleges typically don’t differentiate between AB and BC for admission purpose, but BC in a year (vs. the AB+BC over two years) is definitely harder. It will NOT modify rigor appreciation from the college but, if the high school offers both and the student professes to wish to major in physics, math, or engineering, the college may wonder why the student didn’t take BC. If the student is undecided or is not going into those fields, taking calculus of any kind is a plus (Honors Calc, AB, or BC) at Top 20 schools.
All AP’s show “mastery”. AB is one level of calculus, BC is either the second level of calculus or two levels combined, and is thus harder. It is NOT necessary to take BC unless one intends to major in STEM and even then, taking AB is absolutely fine. Precalc Honors -> AB would be fine if he’s thinking of business or economics, bu Precalc Honors with an A ->BC would be better if he’s aiming for a Top 20 physics, math, or engineering program specifically.