There is no single right answer, and anyone who hands you one without knowing your rate, your reserves, and your temperament is guessing. Start with reserves, ahead of the mortgage. If you do not have a comfortable cash cushion (think several months to a year of living expenses in something liquid), build that first. You can always send money to principal later, but cash sent to the house is hard to pull back out if you lose income. Then compare two returns. Prepaying earns a guaranteed, after-tax return equal to your mortgage rate. Investing earns an expected return, with risk. When your rate is low, say a 3 percent loan, the math leans hard toward investing the difference, since long-run stock returns have historically run well above that, with the caveat being "historically" and no promise about your own holding period. When your rate is high, say 6.5 or 7 percent, prepaying gets attractive, though at a high rate the first move is usually checking whether a refinance makes sense before you commit extra cash. Life stage and discipline settle the rest. Someone a decade from retirement with years left on the loan may reasonably accelerate for peace of mind. And the invest-instead math only works if you will actually stay invested through downturns instead of selling at the bottom. One middle path we like: keep the money liquid in something safe, then make a lump-sum principal payment later and ask the servicer about recasting to reset the payment.