I want to give the most honest possible answer to 'should I pay off debt or invest the difference?'

The mathematical answer: if your debt interest rate is lower than your expected investment return, invest. If it's higher, pay off debt. The crossover is roughly 6-7% for most long-term equity investment expectations.

Practical thresholds:

• Under 4%: invest, no debate. The expected return spread is too wide.
• 4-6%: genuinely ambiguous. Either is reasonable. Personal risk tolerance and psychological preference for debt freedom are legitimate inputs.
• 6-8%: lean toward paying off debt, especially if the debt is not tax-deductible.
• Above 8%: pay off debt aggressively before investing beyond the 401k match.

The exception that always applies: always get the full 401k employer match first, regardless of debt level. The match is a guaranteed 50-100% return that no debt payoff can match.

The answer changes if you factor in tax deductibility. Mortgage interest is sometimes deductible, which lowers the effective rate. Student loan interest up to $2,500 may be deductible. These adjustments matter at the margins.