How do I know when I have enough information to choose between paying debt and investing extra money?

When cash flow and priorities are reviewed together, compare the guaranteed after tax return from paying down the debt with the expected but uncertain return and liquidity of investing. Within the overall strategy, it can be reasonable to split the windfall among debt reduction, reserves, and long term investing. Within the broader financial plan, a low rate loan does not automatically mean investing is better if the household would become anxious or cash poor.