This is one of the most common personal finance questions. The answer depends on your interest rate, risk tolerance, and time horizon.
If your mortgage rate is low, investing in a diversified portfolio may earn a higher return over the long run. If your rate is high, paying down the mortgage gives you a guaranteed, risk-free return.
A good approach is to compare the after-tax cost of your mortgage with your expected investment return. Use the Firephin mortgage calculator to run the numbers.