A 10% discount hands margin to every customer — including those who would have bought at full price. A prize promotion pays only when the trigger event happens, and every non-winner still contributes full margin.
The behavioural physics also differ. Discounts reward what customers already do; prize promises reward what marketers want them to start doing — trial, repeat, higher basket, first visit.
The right question isn't 'prize or discount?' It's 'what behaviour am I paying for, and which instrument buys it most efficiently?'