What would a financial planner model before recommending whether to pay debt or invest extra money?

For this planning decision, compare the guaranteed after tax return from paying down the debt with the expected but uncertain return and liquidity of investing. For the long term plan, it can be reasonable to split the windfall among debt reduction, reserves, and long term investing. As part of the financial analysis, run more than one scenario because a recommendation that works only under one return, inflation, or longevity assumption is fragile.