Whether to prepay mortgage or invest in stocks
In all likelihood, the best thing you can do, if these really are your only two options (ie no other debt at all), paying-down your mortgage will shorten the term of the mortgage, and mean you spend less on your house in the long run. Investing is should be a long-term activity - so yes, the likelihood is that, given a modest investment, it will gain at historical averages over the life of the investment vehicle. However, that is not a guarantee, and is an inherent risk. Whereas paying-down a mortgage lowers your financial obligations and risk, investing increases your risk. I want to know how you got a 2.1% interest rate on a mortgage, though - the lowest I've seen anywhere is 3.25%.