will power is generally thought of as the ability for an organism to use long-term reasoning to resist gratification or short-term rewards. Hypothetically, you might measure inhibitory dampening on primitive drives in the brain (or something...). It's theoretically a measurable quantity.
free will is the idea that some independent entity (like a soul) exists that makes decisions independent of causation. It's not measurable or falsifiable, but lots of evidence suggests no such entity exists.