Whole life and indexed universal life both build cash value you can borrow against, and both get lumped together under high cash value life insurance. Under the hood, though, they run on different engines. Knowing how each one grows tells you a lot about which fits a given situation.
Whole life is the steady one. The insurer credits a guaranteed rate every year, and on top of that a participating policy can receive dividends when the company's results allow. The guarantees are contractual, the growth is smooth, and you always know the floor. The trade is that the ceiling is modest. You're buying certainty and predictability, and the dividend is the upside on top of a floor you can count on.
Indexed universal life ties its crediting to the movement of a market index, with a floor that protects you from negative years and a cap or participation rate that limits the upside in strong ones. In a good stretch it can credit more than whole life. In a flat stretch it can credit near its floor, which is often zero. It also has more moving parts, including internal costs that can rise over time, so it needs monitoring and honest funding to behave.
Neither is simply better. Whole life rewards people who want guarantees and set-and-forget steadiness. IUL can suit someone comfortable with more variability in exchange for a higher potential credit, as long as they'll fund it well and keep an eye on it. The wrong version of either, designed for commission instead of cash value, disappoints for the same reason. Design.
A word of caution that applies to both: dividends and index credits are not guaranteed, only the contractual guarantees are, and policy loans reduce your available cash value and death benefit until repaid. Read every illustration with the guaranteed column as your floor. When you're ready to see what building one looks like, that's Build a Life LOC.