Should I pay off debt or invest?

A practical rule: always capture your full employer 401k match first (it is a 50–100% instant return). Then pay off high-interest debt (generally anything above 6–7%). After that, split money between mid-rate debt and investing. Low-interest debt like a mortgage (3–4%) can reasonably be maintained while investing, since long-term stock market returns historically exceed that rate. Context always matters.